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INTERNATIONAL SERIES OF MONOGRAPHS ON 
PURE AND APPLIED BIOLOGY 

Division: ZOOLOGY 

General Editor: G. A. Kerkut 



Volume 4 



IMPLICATIONS OF EVOLUTION 



OTHER TITLES IN THE SERIES ON PURE AND 

APPLIED BIOLOGY 

Zoology Division 

Vol. 1. RAVEN — An Outline of Developmental Physiology. 

Vol. 2. RAVEN — Morphogenesis: The Analysis of Molluscan 

Development . 
Vol. 3. SAVORY— Instinctive Living. 

Biochemistry Division 

Vol. 1. The Thyroid Hormones. 

Botany Division 

Vol. 1. BOR — Grasses of India, Burma and Ceylon. 

Vol. 2. TURRILL— Vistas in Botany. 

Vol. 3. SCHULTES— Orchids of Trinidad and Tobago. 

Modern Trends in Physiological Sciences Division 
Vol. 1. FLORKIN — Unity and Diversity in Biochemistry. 
Vol. 2. BRACHET — The Biochemistry of Development. 
Vol. 3. GEREBTZOFF— Cholinesterases. 
Vol. 4. BROUHA — Physiology in Industry. 



K '/£ 



IMPLICATIONS OF 
EVOLUTION 

By 

G. A. KERKUT 

M.A., PH.D. 

Department of Physiology and Biochemistry 
The University of Southampton 



PERGAMON PRESS 

NEW YORK • OXFORD • LONDON ■ PARIS 

1960 



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iNTS 



Preface vii 

Acknowledgements ix 

1 INTRODUCTION 1 

2 BASIC ASSUMPTIONS 6 

3 VIRUSES, RICKETTSIAE AND BACTERIA 18 

4 THE PROTOZOA 26 

5 ORIGIN OF THE METAZOA 36 

6 THE MOST PRIMITIVE METAZOA 50 

(1) PORIFERA 54 

(2) MESOZOA 71 

(3) COELENTERATA 76 

(4) CTENOPHORA 84 

(5) PLATYHELMINTHES 94 

7 THE INVERTEBRATE PHYLA 101 

8 BIOCHEMICAL STUDIES OF PHYLOGENY 112 

(1) PHOSPHAGENS 112 

(2) STEROLS 129 

9 VERTEBRATE PALAEONTOLOGY 134 
10 CONCLUSIONS 150 



vi CONTENTS 

Page 
Bibliography 159 

Name Index 169 

Subject Index 171 



PREFACE 



There are many books about Evolution so perhaps in this preface 
I should state what this book is not about. It is not concerned with 
the mechanism of speciation, the evolution of dominance, the 
relationship of enzymatic adaptation to the inheritance of acquired 
characteristics or the probability that Natural Selection can bring 
about a pandemic of rodents in n + 1 years. Instead the present 
book is concerned with an examination of certain basic assumptions 
and implications that have become involved in the present-day 
concept of the evolutionary relationships within the animal 
kingdom. The majority of books on Evolution either blatantly 
treat these assumptions as part of an old (and concluded) historic 
argument or else they avoid discussing the assumptions and 
instead deal with the more scientific and mathematical parts of 
Evolution. 

If one tries to question this avoiding reaction, the protagonists 
round on one and say in an accusing tone of voice, " Don't you 
believe in the Theory of Organic Evolution? What better theory 
have you got to offer? " 

May I here humbly state as part of my biological credo that I 
believe that the theory of Evolution as presented by orthodox 
evolutionists is in many ways a satisfying explanation of some of 
the evidence. At the same time I think that the attempt to explain 
all living forms in terms of an evolution from a unique source, 
though a brave and valid attempt, is one that is premature and 
not satisfactorily supported by present-day evidence. It may in 
fact be shown ultimately to be the correct explanation, but the 
supporting evidence remains to be discovered. We can, if we like, 
believe that such an evolutionary system has taken place, but I 
for one do not think that " it has been proven beyond all reason- 
able doubt." In the pages of the book that follow I shall present 
evidence for the point of view that there are many discrete groups 

vii 



viii PREFACE 

of animals and that we do not know how they have evolved nor 
how they are interrelated. It is possible that they might have 
evolved quite independently from discrete and separate sources. 
There are only a limited number of chemical elements that are 
capable of forming stable polymerisation compounds and it is not 
at all surprising that the same compounds have been formed on 
several occasions. Quite complex materials such as carbohydrates, 
peptides and even nucleic acids can be formed by irradiating water 
containing simple salts and gases. 

It may be suggested that the problem we are examining here, 
namely that of the evolution and interrelationship of the basic 
living stocks is a major problem and one that will test the strength 
and ability of many hundreds of research workers. If this book 
merely indicates to some of the readers that certain lines of thought 
are still open to examination, then I shall consider that it has done 
its allotted task. 

There is, however, a second point that I should like to make, and 
this concerns not factual material but an attitude of mind. It is 
very depressing to find that many subjects are becoming encased 
in scientific dogmatism. The basic information is frequently over- 
looked or ignored and opinions become repeated so often and so 
loudly that they take on the tone of Laws. Although it does take a 
considerable amount of time, it is essential that the basic informa- 
tion is frequently re-examined and the conclusions analysed. 
From time to time one must stop and attempt to think things out 
for oneself instead of just accepting the most widely quoted 
viewpoint. I have dealt with this attitude in the introductory 
chapter of this book, though I hope that the moral does not end 
there but instead runs through the rest of the book as well. 

It is a pleasure to acknowledge the kind help and assistance 
that various colleagues have given me during the writing of this 
work. Many of them have read through parts of the book or 
offered advice on various points. I have profited greatly from their 
counsel, though of course I bear full responsibility for all the 
statements and errors. In particular I should like to thank 
Professor E. Baldwin, Drs. M. S. Laverack, K. A. Munday, 
S. Smith, Miss D. Wisden, Messrs. Robert Walker, Edward 
Munn, and Richard Solly for their help and forbearance. 



ACKNOWLEDGEMENTS 



I am grateful to the following authors and publishers for 
permission to use figures and to quote from their publications: 

Professor G. Schramm for kindly providing the photograph 
from which Fig. 2 was taken. 

Dr. S. Brenner and Dr. R. W. Home for kindly providing the 
photograph from which Fig. 3 was taken. 

Academic Press for permission to quote from the article by 
H. A. Krebs in Chemical Pathways in Metabolism. Vol. 1, edited 
by D. M. Greenberg. 

Allen and Unwin for permission to quote from the article by 
G. R. de Beer in " The evolution of the Metazoa " from the book 
Evolution as a Process edited by J. Huxley, A. C. Hardy and E. B. 
Ford. 

Cambridge University Press for permission to take the table 
printed on page 112 from Comparative Biochemistry by E. Baldwin; 
the table on page 2 from The Cambridge University Handbook; 
and the quotations from Biochemistry and Morphogenesis by J. 
Needham, and Growth and Form by W. D'Arcy Thompson. 
Masson et Cie for permission to reproduce Figs. 7, 8, 9, 11, 12, 15, 
and 16, from Traite de Zoologie edited by P. P. Grasse. 

McGraw Hill for permission to reproduce Figs. 25 and 27, 
taken from The Invertebrates by L. H. Hyman, and also for 
permission to quote from this work. 

Oxford University Press for permission to quote from E. Radl's 
History of Biological Theories and to adapt Fig. 43 from The Horses 
by G. G. Simpson. 

Charles C. Thomas for permission to quote from a paper by 
H. A. Krebs published in the Harvey Lectures. 

John Wiley for permission to take the table on page 20 from 
General Biochemistry by J. S. Fruton and S. Simmons. 

IX 



X ACKNOWLEDGEMENTS 

I should also like to thank the editors of the following journals 
for permission to copy and reproduce material. Archive de Zoo- 
logie General et Experimentale ; Biochemistry, Biophysics Acta; 
Biochemical Journal; Journal of Bacteriology; Journal of Experi- 
mental Biology; Proceedings of the Royal Society; Quarterly Review 
of Biology ; Systematic Zoology and Zoologiska Bidrag. 



' 



1 t 



> 



CHAPTER 1 



INTRODUCTION 



Throughout the Dark and Middle Ages, Learning was under the 
aegis of the Church. Except for useful subjects such as Medicine 
and perhaps Law, the university students were concerned with 
material that would either make the student a useful priest or else 
a person useful to priests. 

The hold that the Church has had on the universities has been 
but slowly relinquished over the years. Until 1871 it was the custom 
for the majority of dons at Cambridge to be ordained before they 
could carry out any of the duties in college. This did not always 
mean that the prospective Fellow had to make a careful study of 
theology. Thus the Fellows of some colleges had the right of be- 
coming ordained in their own chapel as soon as they were elected to 
a Fellowship without having to undergo any arduous extra study. 
This special sanction was taken away from them in 1852 and from 
then on they had to become ordained in the normal manner. 

The Fellows besides being compulsorily ordained also had to 
live under an enforced celibacy. Should they wish to enjoy the 
varied pleasures of married life they had in turn to relinquish 
their college Fellowships. The married clergyman then left 
Cambridge and usually took up one of the livings that were in the 
gift of his college. This had its own compensations ; those scholars 
who had swallowed their intellectual goat in their youth, instead 
of being forced to eke it out to various undergraduates for the rest 
of their lives, could leave Cambridge and take up a rich living in 
the outside world. This made more room available at the university 
for the younger man, who did not then merely have to wait for 
his older colleagues to die. 

The hold of the Church on the university continued in many 
ways. The undergraduates coming up to Cambridge until 1852 

1 



2 INTRODUCTION 

had to be communicants of the Church of England, and the 
undergraduate coming up in, say, 1910 had to satisfy his examiners 
not only in his knowledge of classical languages but also had to 
show that he had some knowledge of Archdeacon William Paley's 
book on Evidences of Christianity. The latter examination was in 
force till 1927, when it was brought to the notice of the university 
authorities that many undergraduates did not in fact read Paley's 
Evidences but instead studied a little crib of them. Many of the 
more sceptical dons in the university were in favour of retaining 
the examination and ensuring that all undergraduates should 
be made to study Paley's Evidences most carefully, " For in this 
way," they said, " the student will be forced to realise just how 
weak the evidence in favour of Christianity really is." This 
argument was not upheld and in 1927 another piece of tradition was 
abandoned. 

Many present-day undergraduates seem to imagine that the 
various subjects they study have existed as such, if not for 
eternity, then at least from time immemorial. They are surprised 
to learn that many of the chairs and examinations only came into 
existence over the last half-century. In the table below I have 
selected a few of the dates at which various chairs became 
established at Cambridge. It will be seen that the subjects of 
Theology and Medicine are very ancient whilst German, French 
and English are relatively modern. 

Establishment of Chairs at Cambridge 

1502 2 Chairs of Divinity 

1540 Civil Law, Physic, Hebrew, Greek 

1634 Arabic 

1683 Moral Philosophy 

1684 Philosophy 

1702 Organic Chemistry 

1704 Astronomy 

1707 Anatomy 

1724 History 

1727 Botany 

1866 Zoology 

1869 Fine Art 

1909 German 



INTRODUCTION 3 

1911 English Literature 

1919 French 

1937 Geography 

1938 Education 

(This is only a selection from the complete list.) 

You may ask, " What has all this got to do with evolution? " 
It is my thesis that many of the Church's worst features are still 
left embedded in present-day studies. Thus the serious under- 
graduate of the previous centuries was brought up on a theological 
diet from which he would learn to have faith and to quote authori- 
ties when he was in doubt. Intelligent understanding was the 
last thing required. The undergraduate of today is just as bad; 
he is still the same opinion-swallowing grub. He will gladly 
devour opinions and views that he does not properly understand 
in the hope that he may later regurgitate them during one of his 
examinations. Regardless of his subject, be it Engineering, 
Physics, English or Biology, he will have faith in theories that he 
only dimly follows and will call upon various authorities to 
support what he does not understand. In this he differs not one 
bit from the irrational theology student of the bygone age who 
would mumble his dogma and hurry through his studies in order 
to reach the peace and plenty of the comfortable living in the world 
outside. But what is worse, the present-day student claims to be 
different from his predecessor in that he thinks scientifically and 
despises dogma, and when challenged he says in defence, "After 
all, one has to accept something, or else it takes a very long time to 
get anywhere." 

Well, let us see the present-day student " getting somewhere." 
For some years now I have tutored undergraduates on various 
aspects of Biology. It is quite common during the course of 
conversation to ask the student if he knows the evidence for 
Evolution. This usually evokes a faintly superior smile at the 
simplicity of the question, since it is an old war-horse set in count- 
less examinations. " Well, sir, there is the evidence from 
palaeontology, comparative anatomy, embryology, systematics and 
geographical distributions," the student will say in a nursery- 
rhyme jargon, sometimes even ticking off the w r ords on his fingers. 
He would then sit and look fairly complacent and wait for a more 

2— IOE 



4 INTRODUCTION 

difficult question to follow, such as the nature of the evidence for 
Natural Selection. Instead I would continue on with Evolution. 

" Do you think that the Evolutionary Theory is the best 
explanation yet advanced to explain animal interrelationships? " 
I would ask. 

1 Why, of course, sir," would be the reply in some amazement 
at my question. " There is nothing else, except for the religious 
explanation held by some Fundamentalist Christians, and I 
gather, sir, that these views are no longer held by the more 
up-to-date Churchmen." 

" So," I would continue, " you believe in Evolution because 
there is no other theory? " 

" Oh, no, sir," would be the reply, " I believe in it because of 
the evidence I just mentioned." 

" Have you read any book on the evidence for Evolution? " 
I would ask. 

" Yes, sir," and here he would mention the names of authors 
of a popular school textbook, " and of course, sir, there is that 
book by Darwin, The Origin of Species." 

" Have you read this book? " I asked. 

" Well, not all through, sir." 

" About how much? " 

" The first part, sir." 

" The first fifty pages? " 

" Yes, sir, about that much; maybe a bit less." 

" I see, and that has given you your firm understanding of 
Evolution? " 

" Yes, sir." 

" Well, now, if you really understand an argument you will 
be able to indicate to me not only the points in favour of the 
argument but also the most telling points against it." 
I suppose so, sir." 

Good. Please tell me, then, some of the evidence against the 
theory of Evolution." 
Against what, sir? " 
The theory of Evolution." 

" But there isn't any, sir." 

Here the conversation would take on a more strained atmosphere. 
The student would look at me as if I was playing a very unfair 



a 
a 






INTRODUCTION 5 

game. It would be clearly quite against the rules to ask for 
evidence against a theory when he had learnt up everything in 
favour of the theory. He also would take it rather badly when I 
suggest that he is not being very scientific in his outlook if he 
swallows the latest scientific dogma and, when questioned, just 
repeats parrot fashion the views of the current Archbishop of 
Evolution. In fact he would be behaving like certain of those 
religious students he affects to despise. He would be taking on 
faith what he could not intellectually understand and when 
questioned would appeal to authority, the authority of a " good 
book " which in this case was The Origin of Species. (It is inter- 
esting to note that many of these widely quoted books are read 
by title only. Three of such that come to mind are the Bible, The 
Origin of Species and Das Kapital.) 

I would then suggest that the student should go away and read 
the evidence for and against Evolution and present it as an essay. 
A week would pass and the same student would appear armed with 
an essay on the evidence for Evolution. The essay would usually 
be well done, since the student might have realised that I should 
be tough to convince. When the essay had been read and the 
question concerning the evidence against Evolution came up, the 
student would give a rather pained smile. " Well, sir, I looked up 
various books but could not find anything in the scientific books 
against Evolution. I did not think you would want a religious 
argument." " No, you were quite correct. I want a scientific 
argument against Evolution." " Well, sir, there does not seem to 
be one and that in itself is a piece of evidence in favour of the 
Evolutionary Theory." 

At this piece of logic the student would sit back and feel that 
he had come out on top. After all, I had merely been questioning 
him whilst he had produced information. 

I would then indicate to him that the theory of Evolution was 
of considerable antiquity and would mention that he might have 
looked at the book by Radl, The History of Biological Theories. 
Having made sure that the student had noted the book down for 
future reference I would proceed as follows. 




' 



CHAPTER 2 



BASIC ASSUMPTIONS 



Before one can decide that the theory of Evolution is the best 
explanation of the present-day range of forms of living material 
one should examine all the implications that such a theory may 
hold. Too often the theory is applied to, say, the development of 
the horse and then because it is held to be applicable there it is 
extended to the rest of the animal kingdom with little or no further 
evidence. 

There are, however, seven basic assumptions that are often 
not mentioned during discussions of Evolution. Many evolution- 
ists ignore the first six assumptions and only consider the seventh. 
These are as follows. 

(1) The first assumption is that non-living things gave rise to 
living material, i.e. spontaneous generation occurred. 

(2) The second assumption is that spontaneous generation 
occurred only once. 

The other assumptions all follow from the second one. 

(3) The third assumption is that viruses, bacteria, plants and 
animals are all interrelated. 

(4) The fourth assumption is that the Protozoa gave rise to the 
Metazoa. 

(5) The fifth assumption is that the various invertebrate phyla 
are interrelated. 

(6) The sixth assumption is that the invertebrates gave rise to 
the vertebrates. 

(7) The seventh assumption is that within the vertebrates the 
fish gave rise to the amphibia, the amphibia to the reptiles, and 
the reptiles to the birds and mammals. Sometimes this is expressed 
in other words, i.e. that the modern amphibia and reptiles had a 
common ancestral stock, and so on. 

6 



BASIC ASSUMPTIONS 7 

For the initial purposes of this discussion on Evolution I shall 
consider that the supporters of the theory of Evolution hold that 
all these seven assumptions are valid, and that these assumptions 
form the " General Theory of Evolution." 

The first point that I should like to make is that these seven 
assumptions by their nature are not capable of experimental 
verification. They assume that a certain series of events has 
occurred in the past. Thus though it may be possible to mimic 
some of these events under present-day conditions, this does not 
mean that these events must therefore have taken place in the 
past. All that it shows is that it is possible for such a change to 
take place. Thus to change a present-day reptile into a mammal, 
though of great interest, would not show the way in which the 
mammals did arise. Unfortunately we cannot bring about even 
this change; instead we have to depend upon limited circum- 
stantial evidence for our assumptions, and it is now my intention 
to discuss the nature of this evidence. 

Non-living into living (Biogenesis) 

This is one of the oldest problems to puzzle man. Is it possible 
for non-living material simply to be turned into living material 
or is some extra " vital " force necessarv? It is reasonablv clear 
that living bodies in many ways use systems similar to those 
present in the non-living world. One of the first barricades 
appeared to fall to Wohler, when he showed by his synthesis of 
urea that there was no very clear distinction between organic 
chemicals and non-organic chemicals. Within recent years we 
have been able to devise systems in w T hich the irradiation of a 
mixture containing water, carbon dioxide and ammonia brings 
about the formation of amino-acids, simple peptides, and 
carbohydrates. However, proteins and nucleoproteins have not 
yet been synthesised under such conditions and these latter com- 
pounds appear to be of great importance in the development and 
maintenance of life. One imagines that the synthesis of these 
substances will merely be a matter of time and application, but 
it will be useful to distinguish the two different methods of 
achieving their synthesis. The first is to try to synthesise them 
under conditions in which we imagine that living things first 
occurred, i.e. to irradiate simple solutions and hope that proteins 



8 BASIC ASSUMPTIONS 

and nucleoproteins will form by random combination. This 
would mimic the conditions under which we believe life originated. 
The second method is to use specialised chemical and physical 
techniques to synthesise proteins and nucleoproteins, and having 
synthesised them, then to place them in their correct structural 
relationship. In this way, the combination of synthetic proteins, 
nucleic acids, lipids and carbohydrates might lead to the forma- 
tion of a simple virus-like compound that could reproduce in 
living cells. The next stage would be the development of an 
artificial solution to maintain the artificial virus. With these steps 
accomplished we should have learnt a great deal about the processes 
taking place in the living body and no doubt we should have dis- 
covered new rules for physics and chemistry, but we could not 
say from our experiments that the living material in the universe 
arose in this way. The results would show that living matter can 
arise by synthetic methods devised in the laboratory, but it 
would still be possible that there were other methods by which life 
actually arose in the universe. For a full discussion of the origin 
of life one should consult the following articles: Oparin (1957); 
Bernal (1954); Pringle (1954); Pirie (1954); Haldane (1954). 

Life arose only once 

The assumption that life arose only once and that therefore * 
all living things are interrelated is a useful assumption in that it 
provides a simple working basis for experimental procedure. But 
because a concept is useful it does not mean that it is necessarily 
correct. The experimental basis for this concept in particular is 
not as definite and as conclusive as many modern texts would have 
us believe. 

Biochemical evidence. Biochemists and comparative physi- 
ologists usually assume that all protoplasm, no matter where it is 
found, has the same fundamental biochemical and biophysical 
processes taking place in it. But even an elementary study of the 
situation shows that there are often many different ways of 
carrying out a simple process in the animal kingdom. One well- 
known example is that of carrying oxygen in solution; various 
substances such as haemoglobin, haemocyanin, haemerythrin and 
chlorocruorin are known to be capable of combining with oxygen. 
But the common possession of a specific blood pigment does not 



BASIC ASSUMPTIONS 9 

indicate any close phylogenetic relationship. Thus though many 
Crustacea have haemocyanin no biochemist or physiologist would 
suggest taking Daphnia out of the Crustacea because it possesses 
haemoglobin. The role of blood pigments has been much studied 
and in particular we accept the varied way in which they are dis- 
tributed throughout the animal kingdom. Even here we do not 
always know their function ; thus we find haemoglobin in the root 
nodules of leguminous plants (Keilin and Wang 1945), where its 
precise function is as yet not known. Plants, however, seem 
capable of synthesising many substances that are often regarded 
as " mammalian " compounds. Nettle stings contain acetyl 
choline, 5-hydroxytryptamine and histamine, and it is probable 
that these have been independently developed by the higher 
plants. 

There are in the world but some ninety elements, and of these 
only a few such as carbon, nitrogen, oxygen, hydrogen, phosphorus 
and sulphur appear capable of forming natural monomers and 
polymers. It is therefore not surprising that these elements are 
united to form compounds such as citric acid or 5-hydroxy- 
tryptamine in widely separated plants and animals. Such a 
synthesis might have occurred independently on many occasions 
by trial and error. It should be remembered that there is no 
Patent Law in the natural world, and though one can simplify 
the situation by use of William of Occam's razor, the careless 
use of such a weapon can at times be suicidal. 

Our ignorance is even greater in other biochemical fields, yet it 
is often stated that all protoplasm shows the same fundamental 
biochemical systems. The most quoted example is the way in 
which protoplasm oxidises carbohydrates to liberate energy. This 
release of energy is obtained through two biochemical cycles, 
the glycolysis cycle (Embden-Meyerhof) and the tricarboxylic 
acid cycle (Krebs). Many of the chemicals present in these two 
cycles have been found in bacteria, protozoa, plants, lower 
metazoa, birds and mammals, and because some of the ingredients 
are present it is assumed that the whole system is present. The 
argument then runs that because the system is very complex, it 
would be too much to expect that each group developed this 
complex system independently and so protoplasm everywhere 
must have had a common origin. 



10 BASIC ASSUMPTIONS 

Krebs in 1948 discussed the universality of the tricarboxylic 
cycle in cells and tissues. He stated, " there is no doubt that yeast 
cells can synthesise succinate in the presence of glucose, and 
citrate in the presence of acetate, but none of the strains of 
baker's and brewer's yeast tested at the Sheffield laboratory was 
found capable of oxidising succinic or citric acids at a significant 
rate under whatever conditions these substances were tested." 
In 1954 he was of much the same opinion: " thus all the enzyme 
systems required for the tricarboxylic cycle are present in yeast 
cells and there can be no doubt that the cycle can take place. . . . 
However, these findings are not decisive evidence for the assump- 
tion that the cycle is the main terminal respiratory process in 
yeasts. ... In many other organisms another terminal oxidation 
mechanism seems to play a major role. Its nature is unknown in 
the case of yeast. It may be a dicarboxylic acid cycle in certain 
bacteria." It now appears that the events that suggested the 
existence of a dicarboxylic acid cycle in bacteria may be better 
explained in terms of a divergence from the tricarboxylic acid 
cycle (Romberg 1958). The system of terminal oxidation in 
yeasts is still obscure. 

In effect, then, the situation in bacteria, yeasts, plants and the 
lower animals is not as simple or clear cut as might be imagined. 
There is more than one pathway for the breakdown of carbo- 
hvdrates, and the glycolysis cycle and the citric acid cycle are but 
two of many that are in the process of elucidation. Thus recently 
a hexose monophosphate shunt has been described as an alterna- 
tive method by which bacteria and many animal tissues break down 
glucose. This, combined with the possibility of an alternative 
terminal oxidation system, enables us to postulate two more 
systems that may be active in tissue metabolism. This view is 
supported by Cohen (1955a), who in his account of alternative 
pathways in carbohydrate metabolism states, " the time is past 
when we uncritically ascribe phenomena in carbohydrate metabol- 
ism to variations in the Embden-Meyerhof scheme." Cohen 
(1955b) also suggests that at least six major pathways for glucose 
metabolism are known and several may exist simultaneously in 
the same organism. 

When one considers the various animals and bacteria that have 
been studied, it becomes quite clear that what we have so far 



BASIC ASSUMPTIONS 11 

examined is equivalent to a small drop in a very vast ocean. It is 
pleasing that so much has already been discovered, but there is 
very little doubt that there is a great deal yet to be discovered about 
carbohydrate metabolism. It is therefore premature to claim that 
the " universal " occurrence of the glycolysis and citric cycles 
is proof of the common origin of life from one source. 

To indicate some of the further biochemical complexities we 
may briefly mention four points. Firstly, it is often stated that all 
living systems use the same twenty or so amino-acids. This is 
a simplification of the known data. At one time it was thought 
that only the L-amino-acids occurred in natural systems, but 
since then a few D-amino-acids have been isolated. The number 
of known natural L-amino-acids has increased with the develop- 
ment of chromatographic techniques. Meister (1957) quotes some 
seventy naturally occurring amino-acids and he points out that 
new ones are being discovered almost every month ! This is a 
result of the application of new techniques to an extended range 
of animal and plant material instead of restricting research to 
mammalian tissues. 

Secondly, there are a large number of bacteria that use aberrant 
biochemical systems. Outstanding amongst these are the sulphur 
bacteria which grow quite well on water, carbon dioxide, phosphate 
and either sulphuretted hydrogen or sulphur. Another bacterium, 
Thiobacillns ferro-oxidans, can in some cases grow on ferrous iron 
under acid conditions which prevent the direct aerobic oxidation 
of ferrous iron. Other bacteria take ammonia and dehydrogenate 
it, or nitrite and oxidise it. There is some argument whether these 
systems are primitive or whether they are advanced and overlaid 
on the basic glycolysis and tricarboxylic cycles (see p. 22). These 
examples indicate that the metabolic systems in the bacteria are 
extremely varied. 

Thirdly, even in the higher Metazoa the distribution of hydro- 
gen acceptor systems such as in the cytochromes, flavoproteins, 
tocopherols, vitamin K, etc., is no more uniform than the dis- 
tribution of blood pigments we mentioned previously. 

A fourth generalisation that has been made about protoplasm 
is that its energetic systems involve the formation and destruction 
of " high energy " phosphorus compounds and the ubiquity of 
the phosphorus-containing compounds in living cells has been 



12 BASIC ASSUMPTIONS 

regarded as further evidence of a common protoplasmic origin. 
At first it was thought that the " high energy " compounds were 
found only in the form of ATP (adenosine triphosphate). But 
further studies have since shown the existence of many other " high 
energy ' nucleoside triphosphates, e.g. guanosine triphosphate, 
cytidine triphosphate and uridine triphosphate. Recently other 
1 high energy ' ' compounds have been discovered which contain 
sulphur, i.e. acetyl coenzyme A. (Lynen 1952, Lipmann 1958). It 
is possible that further " high energy " compounds will be dis- 
covered in the future and this greater variety will make it less 
obvious that all protoplasm uses the same energetic systems. 

Thus on the biochemical side it seems premature to conclude 
that all protoplasm has a common origin just because many 
cells show the components of the glycolysis cycle, citric cycle and 
the " high energy " phosphate compounds. It is likely that the 
protoplasm of different animals will show the presence of other 
schemes for the systematic degradation of carbohydrates and then 
perhaps in time an analysis of these systems will allow us to come 
to further conclusions about the varied metabolism of protoplasm. 

Morphological evidence. A line of argument developed by 
morphologists to show the common origin of living cells is the 
almost universal occurrence of the mitotic and meiotic cycle. Thus 
Grasse (1952) suggests that such a system indicates the mono- 
phyletic origin of present-day animals and protozoa. But as 
Boy den (1953) pointed out, the mitotic cycle is not so fixed or so 
invariable as people imagine. 

There are variations such as the presence or absence of intra- 
or extra- nuclear spindles and the presence or absence of centrioles. 
Thus Amano (1957) suggests that the chromosomes are separated 
by extending fibres in animal cells though a different mechanism 
exists in plant cells. On the other hand Swann (1951) suggests 
that the chromosomes in the Arbacia egg separate because of the 
contraction of fibres. In fact a perusal of Schrader's book Mitosis 
(1953) makes it quite clear that one difficulty in finding a single 
hypothesis to explain the mechanisms of mitosis in all cells is that 
there are a large number of different mechanisms of mitosis. It 
also seems that various tissues synthesise their DNA at different 
stages of the mitotic cycle and that the chromosomes may be 
duplicated at these various stages (Leuchtenberger 1958). It is 



BASIC ASSUMPTIONS 13 

possible that a more detailed examination of mitosis will show that 
it too is a polyphyletic system devised for the successful separation 
of the nuclear material into two equal sets. Whether one could go 
as far as Boyden (1953) and say ,; Under the circumstances the 
widespread occurrence of what is called mitosis or meiosis is no 
proof of real genetic relationships of all such organisms. On the 
contrary the very existence of such mechanisms in organisms 
otherwise so diverse as Protista, Metazoa, and Metaphyta, is 
strongly suggestive of convergence and may thus be interpreted 
with the theory of the strictly polyphyletic origins of the major 
groups of organisms " is another matter in our present state of 
ignorance. 

What then can one conclude about the chemical and physical 
nature of protoplasm? Simply that we have a very great deal to 
learn about it. Modern developments are making it abundantly 
clear that some of our previous concepts are quite inadequate and 
that the picture is very much more complex than previously 
imagined. It would be a great mistake to assume that all is chaos 
and that there are no general common systems, but it would be a 
mistake of equal magnitude to assume that everything is very 
simple and that but one system will be found in all protoplasm. 
From our present viewpoint there would appear to be at least four 
or five different systems which allow a cell to obtain its energy. 
There are minor variations in this pattern and the higher animals 
may show less variation than do the bacteria (though few higher 
animals other than the pigeon and the rat have been studied). 
The picture in no way allows us to dogmatise and state that life 
in all its manifestations shows a common biochemical system 
indicative of a single genesis. The evidence at present does not 
by any means exclude the concept that present-day living things 
have many different origins. 

Polyphyletic origin of life 

If we do not hold that the origin of life was unique, i.e. life is 
monophyletic, there is the alternative point of view. This is that 
living things have been created many times, i.e. polyphyletic. 

There are two ways of considering the multiple origin of life. 
The first is to consider that life is continuously being created all 
the time, i.e. that spontaneous generation is always occurring. The 



14 BASIC ASSUMPTIONS 

second view is that spontaneous generation occurred at some 
finite time in the past but that it is no longer occurring. 

The continuous formation of life de novo. This theory is of 
considerable antiquity and it might be as well to give a brief 
resume of its history. The responsibility for it is usually placed 
at the door of Aristotle. He wrote: " It is quite proved that certain 
fish come spontaneously into existence not being derived from 
eggs or copulation. Such fish as are neither oviparous nor 
viviparous arise all from one of two sources, from mud or from 
sand, and from decayed matter that rises hence as scum; for 
instance the so-called froth of small fry comes out of sandy 
ground. The fry is incapable of growing and of propagating its 
kind, after living for a while it dies away and another creature 
takes its place and so, with short interval excepted, it may be said 
to last the whole year throughout." 

Other biologists gave various recipes for the formation of life 
de novo. Virgil in his Georgics, Book IV, gives the recipe for the 
formation of a swarm of bees from the barren carcass of a dead calf. 
Van Helmont suggested that mice could be formed " if a dirty 
undergarment is squeezed into the mouth of a vessel, within 
21 days the ferment drained from the garment and transformed by 
the smell of the grain, envelops the wheat in its own skin and 
turns into mice." Van Helmont was surprised that mice formed 
in this manner could not be distinguished from mice produced by 
normal sexual breeding. 

The situation became more critical when the experimentalists 
tried to determine whether it was possible to prevent living things 
from appearing in preserved material. The experiments of 
Needham, Pouchet and Bastian all indicated that living things 
still appeared in solutions from which all previous life had been 
removed, whilst Redi, Swammerdam, Vallisneri, Spallanzani, 
Schwann, Pasteur and many others showed that if the experi- 
ments were done very carefully it was possible to preserve soups, 
blood or urine in an atmosphere of oxygen and still get no growth 
of living material. It is not my intention here to discuss this old 
controversy. Full and interesting details can be found in the books 
of Oparin (1957), Singer (1950) and Wheeler (1939). Today 
there are still people who think that living things of a high level 
of complexity can be formed de novo. Of these it is perhaps of 



BASIC ASSUMPTIONS 15 

interest to quote from one Wilhelm Reich (1948). Reich has 
developed the concept that living material accumulates units of 
primordial energy which he calls " orgones." These orgones may 
be taken up by small vesicles (bions) that exhibit certain similar- 
ities to living material. By studying these bions and bion complexes 
under the very high optical magnification of 5,000 times (" it is 
not a matter of visualising finer structural detail but movement ") 
Reich concludes that bacteria and Protozoa can arise from sterilised 
organic and inorganic material. Thus from autoclaved grass he 
observed the development of amoebae and other Protozoa. Reich 
was not satisfied with the alternative explanation that the spores 
might have been present in the grass since he had also obtained 
similar amoebae from inorganic material such as sand or iron 
filings placed in the sterilised medium! The bions give off 
radiations which affect living material, and in some ways this 
radiation resembles the mitogenetic radiation studied by Gurwitsch 
(1926). It will be remembered that Gurwitsch claimed that the 
mitogenetic rays which come off from living cells affect the division 
rate of other cells. The experimental verification of mitogenetic 
radiation has proved to be very difficult and at best inconclusive ; 
the evidence is summarised in Hollaender and Schoeffel (1931) 
and in Gray (1931), but as yet there has been no work on orgones 
other than from Reich and his colleagues. The work of Reich is 
of interest in that it shows that there are still " heretics " at work 
on the age-old problem of the origin and nature of living organisms. 
Joseph Needham, writing in his textbook Chemical Embryology 
(1931) stated, " It may be remarked here, without irrelevance, 
that the problem (of spontaneous generation) is still unsolved ; for 
all that was proved by the experiments of Spallanzani was that 
animals the size of rotifers and Protozoa do not originate spontane- 
ously from broth, and all that was proved by those of Pasteur 
was that organisms the size of bacteria do not originate de novo. 
The knowledge which we have acquired in recent years of filter- 
passing organisms such as the mosaic disease of the tobacco plant, 
and phenomena such as the bacteriophage of Twort and d'Herelle 
has reopened the whole matter, so that of the region between, for 
example, the semi-living particles of the bacteriophage (10~ 15 g) 
and the larger-sized colloidal aggregates (10~ 18 g) we know 
absolutely nothing. The dogmatism with which the biologist of 



16 BASIC ASSUMPTIONS 

the early twentieth century asserted the statement omne vivum 
ex vivo was, therefore, like most dogmatisms, ill timed." 

The argument developed so far, then, is as follows. The 
ancients thought that in many cases it was possible for living things 
to be created de novo. All these cases depended upon poor 
observation or lack of knowledge, and gradually as information has 
become available all the higher animals have been shown to arise 
from previous generations. The simpler forms of life such as 
yeast and bacteria were at one time thought to arise spontaneously, 
but controlled experiments showed that these observations were 
at fault. The conclusion is thus that the onlv cases where we think 

j 

that life may be formed de novo are those where we have no 
information as to the mode of origin. From this one might suppose 
that spontaneous generation does not take place, but this is an 
unjustified extrapolation. The correct extrapolation would be 
that until we have devised experiments in which the simpler forms 
of life, such as viruses, are developed de novo, we have no evidence 
of de novo origin of life. This does not imply that de novo genera- 
tion is or was impossible: Oparin (1957) suggests that life was 
created de novo on this world at one time and it is possibly being 
created now somewhere in the universe, but it is not being 
created now in this world since the ubiquitous presence of living 
bacteria would prevent the accumulation of the necessary raw 
materials for the formation of life de novo. When life was first 
created there were no such bacteria and hence the necessary 
substances accumulated. If we accept Oparin's view that life is 
not formed de novo at present in the world, there are still two 
alternative suggestions concerning the origin of life. The first is 
that life is still being formed de novo in other parts of the universe 
and is then transmitted by meteorites to this planet. Ousdal 
(1956) has described in meteorites some very interesting shapes 
which in some ways resemble present-day living forms (Fig. 1). 
However, the meagre evidence so far available that meteorites may 
contain living material is not yet convincing. It should be noted 
that the present climate of opinion concerning the possible 
mechanism of the evolution of the present solar system is changing. 
The view suggested by Sir James Jeans that the planets were 
formed by the unique passage of a giant star near to the sun is 
no longer strongly supported (Lyttelton 1956). Instead it seems 



BASIC ASSUMPTIONS 17 

that solar systems similar to our own have been created many 
millions of times and thus conditions favourable to life may be 
present on many other planets in the universe. Shapley (1957) 
has calculated that there are probably 10 8 planets that have con- 
ditions favourable for life of one sort or another. We have no 
evidence that living things can be transmitted between the stars 
and still remain alive on reaching their destination, but it would 
seem that we shall shortly have information on transmission of 
living material by rockets. It would perhaps be more to the point 
to have information concerning transmission by meteorites. 
Oparin (1957) gives quite an extended discussion of meteorite 
transmission of life and concludes that it was, and still is, highly 
improbable. Nevertheless, at present we have very little informa- 
tion on this subject and it is likely that the renewed interest in 
space travel will stimulate further investigation into the nature and 
properties of meteorites. 

Unique occurrence of life. The second suggestion is that 
though we are unable to show at present that life is formed de 
novo on this earth, there is no evidence to show that when life was 
formed on this earth it was a unique event. Haldane (1954) and 
Oparin (1957) are of the opinion that life was uniquely formed, but, 
as they both point out, nothing is definitely known about what did 
happen; all is hypothesis, and though it is simpler to assume 
that it was a unique occurrence there is no reason why this simple 
explanation should be the correct one. In the previous pages it 
has been pointed out that our knowledge of the cell metabolism 
is insufficient to allow us to state categorically that all cells in all 
living forms have the same biochemical systems at work. Though 
the similarities are often great, the dissimilarities may be just as 
impressive. 

If living material had developed on several different occasions 
or at different places at the same time, then one would expect to 
have a large number of distinct groups of animals, whose relation- 
ships and affinities are difficult to determine. 

This, as we shall see, is the present situation. 



CHAPTER 3 



VIRUSES, RICKETTSIAE AND BACTERIA 



If one assumes that the origin of life was a unique occurrence 
then it follows that all the present-day living things must be 
derived from this original source. This then poses the problem, 
" What is the relationship between the present-day forms? ' In 
many cases it is difficult to form any definite conclusion 
regarding these relationships and this certainly seems to 
hold for the relationship between Viruses, Rickettsiae and 
Bacteria. 

The viruses 

The viruses are of interest since they show many of the 
properties of living material. At first they were described as 
material that would pass through a bacterial filter and which w T as 
capable of reproducing in the living cell. But later on consider- 
able confusion arose over the chemical nature of viruses, the main 
trouble being one of over-simplification. Many people thought 
that the viruses were necessarily simple because they had been 
prepared in a crystalline condition. This concept was furthered 
when chemical analysis showed that the virus was composed 
of a " simple chemical substance " — nucleoprotein. With more 
advanced techniques it became clear that there was considerable 
variability in virus structure. Markham, Smith and Lea (1942) 
showed that when the tobacco mosaic virus was irradiated, only 
a small part of the virus proved sensitive to radiation. This part 
was some 5%-6% of the virus area and in effect it behaved like 
the nucleus of the virus. In 1951 Markham and Smith presented 
evidence that the turnip mosaic virus contained at least two 
distinct components, a nucleic acid component (38%) and a 
structural protein component (62%). 

18 




Fig. 2. Virus structure. The tobacco mosaic virus is made up from 
at least two components. There is a central rod of nucleic acid and 
a series of units of protein that fit over the central rod. In the 
photograph shown here part of the second component has been 
dissolved away to reveal the nucleic acid. (This photograph was 
obtained through the kindness of Professor G. Schramm.) 





Head 



Contracted 
sheath 



r 



ore 



ft Plate 



I art 

fibre 



Fig. 3. Virus structure — Bacteriophage. The Bacteriophage has a 
more complex structure than a simple virus such as the tobacco 
mosaic virus. It has a well-developed head and a tail. The head 
contains the nucleic-acid component and this flows through the 
tail into the bacterium it attacks and there reproduces. (This 
photograph was obtained through the kindness of Dr. S. Brenner 

and Dr. R. W. Home.) 




Fig. 4. Bacterial structure. The bacterial cells have complex 
structure as is shown by this electronmicrograph of a section 
through B. cereus. The cells have a well-defined cell wall, a 
nuclear structure, and many types of cell inclusions. (From 

Chapman and Hillier.) 

(A) Cell wall. (F) Fibrous material. 

(C) Peripheral bodies. (G) Nucleus. 

(D) Transverse cell wall. (H) Cytoplasm. 

(E) Transverse cell wall. (|) Inclusions. 

L.M.R. Limit of light microscope. 



VIRUSES, RICKETTSIAE AND BACTERIA 19 

Takahashi and Ishii (1953) showed that it was possible to find 
the structural protein in the sap of the plants infected with the 
mosaic virus and that it differed from the normal plant proteins. 
This protein had no power of reproduction but required the 
presence of nucleic acid. If the nucleic acid was added to the 
structural protein, then it became capable of reproduction inside 
the cell (Fraenkel-Conrat and Williams 1955). 

The chemical analyses of virus structure have been paralleled 
by studies using the electron microscope. These show that the 
tobacco mosaic virus is often found in rod-like forms, the rods 
being made up of a series of discs each with a hole in the centre. 
The hole is apparently filled with the nucleic acid whilst the disc 
itself is probably the structural protein (Fig. 2). 

Other viruses such as bacteriophage which attacks bacteria 
have an even more complex structure. The bacteriophage has a 
tadpole-shaped head and a small tail (Fig. 3). The head consists 
of a shell of structural protein inside which is the nucleic acid. 
Hershey (1956) described how it was possible to remove the 
nucleic acid from the bacteriophage and leave the tail and the 
shell. This skeleton was still capable of attacking a bacterium 
and killing it, but it was not capable of self-reproduction. 

Detailed chemical analysis and electron microscope studies 
have therefore shown that viruses are not simple single chemical 
substances. There is a considerable range of structural and 
chemical complexity within the group of viruses and it is possible 
to draw up a table showing the differences in their chemical 
composition. 

Material Present Virus 

RNA 
DNA 

Protein S- Animal virus 

Fats 

Carbohydrates J 



i 



RNA 

DNA ? 

Protein Y Bacteriophage 



Fats 

3— IOE 



20 VIRUSES, RICKETTSIAE AND BACTERIA 

Material Present Virus 

RNA "| 

DNA ^ Polyhedral virus 

Protein J 



Tobacco mosaic virus 



RNA \ 

Protein j 

RNA ? 

Thus the analysis of the vaccinia virus shows the presence of 
proteins, DNA, neutral fat, phospholipid, cholesterol, biotin, 
flavine, copper and various as yet unidentified substances. The 
Lee influenza virus has about 5% of its weight as a complex 
polysaccharide containing mannose, galactose and glucosamine. 
It is also becoming clear that the term " nucleic acid " should be 
used with care since there are many different nucleic acids, and as 
ChargafT (1957) points out, often the term " ribose nucleic acid " 
is used when there is no evidence that the sugar ribose is present. 
The precise structure of the nucleoproteins is not yet known, i.e. 
the type of proteins, and the way in which the nucleic acid is 
attached to the protein have yet to be fully elucidated, but some 
evidence is available concerning the component nucleotides in the 
nucleic acids of the virus. The table below, taken from Fruton 
and Simmons (1958) indicates that the proportional composition 
of the nucleotides varies in the different viruses. 

Virus Molar proportions in nucleic acid 

Adenylic Guanylic Cytidylic Uridylic 
acid acid acid acid 

Tobacco mosaic virus 1-0 0-89 0-65 0-88 

Cucumber mosaic virus 1-0 1-0 0-75 1-15 
Tomato bushy stunt 

virus 1-0 10 0-74 0-89 
Turnip yellow mosaic 

virus 1-0 0-76 1-68 0-98 

This gives some hint of the complexity of nucleic acid, and 
nucleoprotein structure, and makes one careful when ascribing 
simplicity to a system that is not yet adequatel)' understood. 



VIRUSES, RICKETTSIAE AND BACTERIA 21 

There are two main views concerning the nature of viruses. One 
suggests that they are in fact the simplest and most primitive 
forms of living material and that originally they utilised the pro- 
teins found in the complex primaeval " soup." As they gave rise 
to more complex living things which altered and destroyed the 
primaeval soup, so they became obligate parasites in other living 
systems that evolved along different lines. On the other hand there 
is the view that the viruses arose from more complex systems and 
that in effect they are more like genes that have taken on a free- 
lance life. Both of these views are discussed by Luria (1953). 

There are other opinions concerning the nature of viruses. Thus 
Hadzi (1953), for example, has suggested that viruses are the 
spores of parasitic Protozoa. It is possible that all these opinions 
are correct and that the viruses are a complex group of substances 
at present classified by their properties and that these properties 
depend on the level of organisation that has been achieved. The 
viruses are thus most likely a grade of organisation that has been 
reached from many different directions. 

In this context and throughout the book, a grade may be re- 
garded as a group of individuals that are united by certain common 
properties but are not derived from a common close ancestor. 
The grade indicates the level of organisation rather than a close 
phylogenetic relationship. 

The rickettsiae 

The rickettsiae cause such diseases as typhus, murine fever and 
spotted fever. They have properties between those of bacteria 
and viruses; they approach the bacteria in structural complexity 
and size, and they resemble viruses in that they are unable to 
reproduce outside living cells (though this is not a stringent 
criterion; it merely indicates lack of experimental success so 
far). 

The rickettsiae are more complex than viruses in that they are 
able to carry out certain of the metabolic processes of the higher 
cells. Thus they are capable of oxidising glutamate, pyruvate, 
succinate, fumarate and oxalo- acetate. These substances are also 
oxidised by the mitochondria of the normal cell, and the sug- 
gestion has been made that the rickettsiae are in fact free mito- 
chondria. Thus both the mitochondria and rickettsiae lose their 



22 VIRUSES, RICKETTSIAE AND BACTERIA 

diphosphopyridine nucleotide and coenzyme A on freezing, the 
freezing in some way affecting the properties of the membrane 
around the rickettsiae or mitochondria. It is possible that the 
rickettsiae are developed as free mitochondria and that the viruses 
are further simplifications. On the other hand the rickettsiae 
may indicate a stage in the development of the viruses to bacteria 
or the three groups could be quite unrelated. 

We have insufficient evidence as yet to come to any firm con- 
clusion concerning the origin and affinity of the rickettsiae. 

Bacteria 

We are no wiser when we come to consider the status of the 
bacteria. Within recent years there has been a considerable 
increase in our knowledge of the structure of the bacterial cell 
(Spooner and Stocker 1956; Zinsser 1957). Thus Robinow in 1946 
suggested that there were certain components within the cells of 
Escherichia coli that behaved like nuclear material during cell 
division (Fig. 4). Lederberg (1947) showed that a type of crossing 
over occurred between certain strains of E. coli and that in effect 
it was possible to draw up a map of the positions of various factors 
in bacterial metabolism. The conclusion, then, is that certain 
bacteria show nuclear and sexual (parasexual) behaviour. On the 
other hand there are many bacteria that do not show these 
phenomena, their structure and life history being much more 
simple. 

It is not clear whether the bacteria represent an evolutionary 
approach to the Protozoa, whether they are a retreat from the 
Protozoa or whether they are quite unrelated. Perhaps some of the 
difficulties can be illustrated by considering the autotrophic 
bacteria (Chemoautotrophic) (Fry and Peel 1954). These 
bacteria such as the sulphur and iron bacteria are able to metabolise 
various simple substrates. They raise the question " are these 
bacteria using a more primitive (earlier developed) system than 
those found in the heterotrophic and photosynthetic bacteria? ' 
It is not possible to give a definite answer to this question since 
our knowledge of the biochemistry of the heterotrophic and 
chemoautotrophic bacteria is still very incomplete. The chemo- 
autotrophs can obtain their energy from simple sources such as 
hydrogen, methane, ammonia, nitrite, hydrogen sulphide or iron 



VIRUSES, RICKETTSIAE AND BACTERIA 23 

compounds. These substances are very much less complex than 
the carbohydrates from which the higher animals obtain their 
energy. The simple hypothesis is that the chemoautotrophs are a 
side-line representing a more primitive state of development than 
that shown by heterotrophic and photosynthetic bacteria. Though 
this opinion is quite widely held, evidence is gradually accumula- 
ting to indicate the opposite view; viz. that the chemoautotrophs 
are in fact using systems that are secondarily simplified from those 
of the heterotrophs. Thus O'Kane (1941) showed that the 
sulphur bacterium Thiobacillus thioxidans could synthesise various 
vitamins of the B group. These substances are used mainly in 
normal heterocyclic heterotrophic glycolysis; thiamine is used in 
oxidative decarboxylation; riboflavine is a coenzyme for the 
hydrogen acceptors, nicotinic acid forms part of Coenzymes I 
and II. It would therefore be interesting to know what role they 
plav in Thiobacillus. It would appear that the bacterium has many 
of the enzymes that are used in heterotrophic glycolysis but that 
it uses special variations on the normal system. The chemo- 
autotrophs would then have superimposed their own system upon 
that of the heterotrophs. 

A schematic system for the development of metabolic systems is 
shown below. If this is correct, and the chemoautotrophs are 
less primitive than the heterotrophs, it again points the lesson 
that the simplest explanations are not necessarily the correct 
ones. 

Scheme for the origin of metabolic systems (after Oparin) : 

(1) Solution containing salts. 

(2) Solution containing salts and simple organic compounds. 

(3) Solution containing salts, simple and complex organic 
compounds. 

(4) System that turns complex materials into simple organic 
materials and so obtains energy. Also able to reproduce 
itself '■= a living system. 

HETEROTROPHS (only glycolysis cycle) 

(5a) Living system that converts complex organic material to 
simple material. 

HETEROTROPHS (glycolysis and citric cycles. 
Hydrogen acceptors) 



24 VIRUSES, RICKETTSIAE AND BACTERIA 

(5b) Living system that converts simple material to obtain 
energy. 

CHEMOAUTOTROPHS (attack H 2 S,CH 4 , etc.) 

(5c) Living system that develops PHOTOSYNTHESIS. 

(Note that animals can by chemical means build up C0 2 
to form carbohydrates.) 

Bacteria and Protozoa 

It is problematical how, if at all, the Bacteria are related 
to the Protozoa and, if so, which Bacteria gave rise to which 
Protozoa. 

Grasse (1953) thinks that the Protozoa are in fact monophyletic 
and derived from the Bacteria. He bases this opinion on the 
following resemblances between the Protozoa and Bacteria. 

(1) Both have vacuoles. 

(2) Both contain proteins, lipids and carbohydrates. 

(3) Both have mitochondria. 

(4) Certain bacteria have a nucleus and chromosomes. 

(5) A sexual process has been described in some bacteria. 

(6) Both can possess flagella. 

(7) Spore formation occurs in both. 

(8) The membranes around the cell in each case are sometimes 
morphologically similar. 

These resemblances are rather tenuous and not all apply to any 
one bacterium. In effect it is difficult to know to what extent the 
resemblances are real phylogenetic ones and to what extent they 
have risen by convergence. Thus the bacterial flagellum is very 
much more simple in structure than the protozoan flagellum. The 
protozoan flagellum has an inner strand of two rods and an outer 
ring of nine rods. The bacterial flagellum has just the inner 
strand of one or two rods. Until we know a great deal more 
about the electron microscopy of the bacterial and protozoan cells 
we shall not be in any position to base relationships on morpho- 
logical similarities. 

The relationship between Bacteria, Protozoa and Metazoa has 
been discussed by Grasse, who derives the following alternative 
systems : 



VIRUSES, RICKETTSIAE AND BACTERIA 

Autotrophic bacteria ->- Protophyta ->Metaphyta 

(i) i _ i 

Heterotrophic bacteria Protozoa — > Metazoa 



25 



(2) 



Autotrophic bacteria -> Protophyta -> Metaphyta 
Heterotrophic bacteria -> Protozoa -> Metazoa 



Autotrophic bacteria Protophyta ->- Metaphyta 

(3) I t 

Heterotrophic bacteria — > Protozoa ->■ Metazoa 



Autotrophic bacteria - 

(4) } 

Heterotrophic bacteria — > Protozoa 



Protophyta ->■ Metaphyta 
— > Metazoa 



Grasse suggests that the first system is the most probable but that 
all the others have something to be said in their favour. On the 
other hand Oparin (1957) thinks that the heterotrophic bacteria 
are the most primitive and that they gave rise to the autotrophic 
forms and to the Protozoa and Protophyta, a view that is not 
mentioned in Grasse's scheme. It can be seen that nothing is 
definite ; all is hypothesis and opinion. 

At present the following schemes all seem equally likely: 

(1) The Bacteria and the Protozoa had an independent 
phylogenetic origin. 

(2) The Bacteria are more primitive than the Protozoa and gave 
rise to the Protozoa. 

(3) The Bacteria are secondarily simplified and were derived 
from the Protozoa. 

(4) The Bacteria are a polyphyletic grade. Some are more 
primitive than the Protozoa, others are derived from the 
Protozoa. 

We have at present insufficient evidence to enable us to choose 
between these hypotheses. 



CHAPTER 4 



THE PROTOZOA 



We have just seen that the relationship between the simplest 
living forms, the Viruses, Rickettsiae and Bacteria, is not at all 
clear. We cannot say with any certainty how they have evolved 
and what the relationship is between the three groups. When we 
come to consider the next group of animals, the Protozoa, we shall 
find a very similar situation. There is great difficulty in deciding 
where the Protozoa came from, what they gave rise to, and what 
their interrelationships are. The Protozoa can be classified into 
four classes. These are: 

(1) Flagellata; e.g. Chlamydomonas, Trichonympha. 

(2) Rhizopoda; e.g. Amoeba, Elphidium. 

(3) Sporozoa; e.g. Monocystis, Plasmodium. 

(4) Ciliophora; e.g. Paramecium, Entodinium. 

There are several evolutionary problems to be found in the 
protozoans but only three of these will be considered here. The 
first problem is, " Which of the four classes is the most primitive? ' 
The second problem is, " What is the interrelationship of the four 
classes? " and the third problem is, " What is the status of the 
group Protozoa? ' These problems will each be examined in turn. 



The Most Primitive Protozoa 

We can readily dismiss two of the four classes of the Protozoa 
as candidates for the position of the most primitive class. The 
Sporozoa are almost entirely parasitic in the bodies of higher 
animals and they spend their life in the outside world in the 
encysted state. Though it is possible that they could have arisen 
in the " primaeval soup " that gave rise to living forms, their life 

26 



THE PROTOZOA 27 

cycles are so involved and their structure with myonemes and spore 
cases so complex that they are probably not very close to the 
primitive stock. It is worth noting here that some authors such as 
Ulrich (1950) have suggested that the Cnidosporidia are not 
protozoans but metazoans. 

The Ciliophora too can be disregarded since they show par 
excellence the extremely complex structures that can exist within 
the protozoan cell. Thus Entodinium with its complex cirri, 
neuromotor system, skeleton, nuclei and digestive system is 
almost as complex as some metazoans (Fig. 5). On the other hand 
even the most simple of the ciliophorans have a complex nuclear 
structure. There is usually a macro- and a micro-nucleus as 
separate bodies, though they are in the form of macro- and micro- 
chromosomes in a single nucleus of the Chonotricha such as 
Spirochona. In addition there is the very complex infraciliature 
that has developed in the superficial regions of the ciliates and 
this is more complex than that found in the flagellates. 

This then leaves two classes, the Flagellata and the Rhizopoda, 
as the more primitive protozoans and each of these has at various 
times been considered as the most primitive Protozoa. Thus at 
the beginning of the century the prevalent view was that the 
Rhizopoda were the most primitive of the Protozoa. This view 
was well expressed by Ray Lankester (1890) in his article in the 
Encyclopaedia Britannica. Lankester said, ' Briefly stated the 
present writer's view is that the earliest protoplasm did not 
possess chlorophyll and therefore did not possess the power of 
feeding on carbonic acid. A conceivable state of things is that a 
vast amount of albuminoids and other such compounds had been 
brought into existence by those processes which culminated in the 
development of the first protoplasm, and it seems therefore likely 
enough that the first protoplasm fed on these antecedent steps in 
its own evolution just as animals feed on organic compounds at 
the present day, more especially as the large creeping plasmodia of 
some Mycetozoa feed on vegetable refuse. It is indeed not 
improbable that, apart from their elaborate fructification, the 
Mycetozoa represent more closely than any other living forms the 
original ancestors of the whole organic world. At a subsequent 
stage in the history of this archaic living matter chlorophyll was 
evolved and the power of taking carbon from carbonic acid. The 



28 



THE PROTOZOA 



Motorium | 



dorsal disc 



Mouth 



Frontal membranellae— 



Contractile vacuole 



Meganucleus' 



Micronucleust 



Cuticl 



Ectoplasm 



Contractile vacuole- 




oral cirri 



oesophagus 



retractor fibres 



Endoderm 



Food vacuoles 

- Caecum 
- Retractor fibres 



Anus 



Fig. 5. Protozoan structure. — Entodinium. This is a complex 
ciliate and has much of the differentiation that one expects to 
find in the higher animals. Thus it has a mouth, gullet, cloaca, 
myonemes, neuronemes, contractile vacuoles, skeletal system and 
several nuclei. (After C. V. Sharp.) 



THE PROTOZOA 29 

1 green ' plants were rendered possible by the evolution of 
chlorophyll, but through what ancestral forms they took their 
origin or whether more than once, i.e. by more than one branch, 
it is difficult even to guess. The green Flagellate Protozoa 
(Volvocinae) certainly furnish a connecting point by which it is 
possible to link on the pedigree of green plants to the primitive 
protoplasm; it is noteworthy that they cannot be considered as 
very primitive and are indeed highly specialised forms as compared 
with the naked protoplasm of the Mycetozoon's plasmodium. 
Thus we are led to entertain the paradox that though the animal is 
dependent on the plant for its food yet the animal preceded the 
plant in evolution, and we look among the lower Protozoa and not 
among the lower Protophyta for the nearest representatives of that 
first protoplasm which was the result of a long and gradual 
evolution of chemical structure and the starting point of the 
development of organic form." 

If one consults any of the older texts such as those of Lankester 
(1909), Delage and Herouard (1896) or Kukenthal and Krumbach 
(1923) one finds that the Rhizopoda are placed as the first class of 
the Protozoa. 

The accent of protozoan research changed during the first 
part of the twentieth century. Instead of being concerned with the 
morphology and life cycles of the Protozoa, the interest became 
more centred upon the physiology and in particular the nutritional 
requirements of the Protozoa. This change in accent from a 
morphological one to a physiological one may explain the change 
that took place in the prevalent attitude to the phylogeny of the 
Protozoa. In such texts as those of Hyman (1940) or Grasse 
(1952) the Flagellata take pride of place over the Rhizopoda; the 
Flagellata being the first class to be described. It should, however, 
be noted that Klebs in 1892 suggested that the Flagellata were in 
fact more primitive than the Rhizopoda. 

Grasse points out that since many of the members of the 
Flagellata possess chlorophyll they are able to undertake synthesis 
of all their food requirements without the assistance of any 
complex compounds. This view is much the same as that of 
Pringsheim (1948), who showed that many of the colourless 
flagellates such as Astasia or Polytoma can be found in pure 
cultures of Euglena and Chlamydomonas respectively. The 



30 THE PROTOZOA 

coloured forms gave rise to the colourless forms, i.e. Astasia is a 
colourless Euglena, and hence the groups Astasia and Polytoma 
are not strict monophyletic genera but instead are polyphyletic 
grades. Pringsheim suggests that many of the present-day colour- 
less flagellates are derived from the coloured form and this would 
make the coloured forms more primitive than the colourless forms 
(Pringsheim and Hovasse 1950). 

Lwoff (1944) in his book on physiological evolution goes even 
further and contends that from a physiological point of view 
evolution is retrogressive. The most primitive Protozoa, he states, 
must surely have been entirely self-supporting with little or no 
food requirements, but as evolution occurred the cells lost their 
synthetic ability and became more and more dependent upon other 
cells for the provision of their food requirements; i.e. they 
regressed instead of progressed. 

There appears to be a fallacy in LwofT's argument. The fact 
that a cell has minimal food requirements does not mean that this 
is necessarily the most primitive condition. In fact most of the 
schemes suggested for the origin of living material place the 
advent of chlorophyll at a very late stage in the evolutionary 
sequence, the plant cells having the chlorophyll system super- 
imposed on the anaerobic metabolic system (Oparin 1957). The 
very earliest living forms would have had considerable food re- 
quirements. It would be perfectly possible for a sarcodine-like 
form to be the most primitive animal feeding on amino-acids and 
carbohydrates synthesised by abiogenic methods. The presence 
of chlorophyll is indeed a good reason for considering the Flagellata 
as an advanced group of the Protozoa. This view was in fact 
suggested by Lankester in 1909. 

Lankester stated, " The real question ... is whether we find 
reason to suppose that the combination of carbon and nitrogen 
to build up proteid, and so protoplasm, required in the earliest 
state of the earth's surface, the action of sunlight and the chloro- 
phyll screen. We must remember that these are now necessary 
for the purpose of raising carbon, and indirectly nitrogen, from the 
mineral resting state to the high elaboration of the organic molecule, 
yet it is, after all, living protoplasm which effects this marvel with 
their assistance ; and it seems (though possibly there are some who 
would deny this) that it is protoplasm which has, so as to speak, 



THE PROTOZOA 31 

invented or produced chlorophyll. Accordingly I incline to the 
view that chlorophyll as we now know it is a definitely later 
evolution — an apparatus to which protoplasm attained, and as a 
consequence of that attainment we have the arborescent, filamentous, 
foliaceous, fixed series of living things we call plants. But before 
protoplasm possessed chlorophyll it had a history. It had in the 
course of that history to develop the nucleus with its complex mecha- 
nism of chromosomes, and it had during that period to feed." 

There is a second reason why the Flagellata are sometimes 
considered to be more primitive than the Rhizopoda. During their 
young stage some of the Rhizopoda such as Naegleria and 
Dimorpha show a flagellate condition which is considered by some 
investigators to be a form of recapitulation; i.e. the young stage 
shows more primitive characteristics than those present in the 
adult. How much faith can one have in this type of argument? 
We know that certain flagellates such as Mastigamoeba show 
pseudopodia as well as flagella and thus the presence or absence of 
pseudopodia or flagella does not necessarily indicate primitive- 
ness. What is more important is the concept that these Rhizopoda 
show the flagellate stage only in the young forms and that there- 
fore the flagella are more primitive than the pseudopodia. 

We are lucky in that there has been a recent investigation by 
Willmer (1956, 1958) into the factors that determine the acquisi- 
tion of flagella by the amoeba Naegleria gruberi. Willmer showed 
that the amoeboid Naegleria can be made to turn into a flagellate 
form with one to four flagella by placing it in water. The change 
takes from 20 min to 24 hr to complete and during this time 
is accompanied by the development of a definite antero-posterior 
axis in the cell; the flagella appearing at the anterior end. The 
pseudopodia can develop from any part of the animal. The 
presence of salts such as lithium chloride, magnesium chloride 
and magnesium sulphate suppress the development of the 
flagella but leave the pseudopodia fully active. The change from 
amoeboid to flagellate condition is reversible and depends upon 
the environmental conditions (Fig. 6). This means that the 
flagellate condition is not necessarily found in the young animal ; 
either stage can reproduce and either stage can be found in the 
young animal. Bunting (1926) showed that the rhizopod Tetramitus 
could undergo cell division in either the amoeboid or the flagellate 



32 




THE PROTOZOA 







Fig. 6. Naegleria gruberi. This protozoan can exist in either of two 
forms: an amoeboid form or a flagellate form. Stages 1-8 show- 
stages during which the amoeboid form changes into the flagellate 
condition. The arrow indicates the direction in which the animal 

moves. (From Willmer.) 

stage. This too indicates that the flagellate stage is not necessarily 
the more juvenile one. 

On the basis of this evidence we are left undecided as to which 
is the most primitive, the Flagellata or the Rhizopoda. This 
question will be dealt with again on p. 33, where a third inter- 
pretation w r ill be presented. In effect this third view states that the 
Rhizopoda and Flagellata are not strict classes of the Phylum 
Protozoa. Instead they are polyphyletic grades. The Flagellata arose 
on many separate occasions from the plants, fungi and metazoa, and 
the Rhizopoda developed in much the same manner. Both these 
groups are then more in the nature of horizontal grades than vertical 
monophyletic classes, one of which is older than the other. 

Protozoan Phylogeny; the Interrelationship 
of the Four Classes of Protozoa 

The precise relationship of the four classes of Protozoa is 
uncertain. The two classes that appear to be the most closely 
related are the Flagellata and the Rhizopoda. Butschli in 1883 



THE PROTOZOA 33 

suggested that it was possible to derive these two classes from 
intermediate forms such as Mastigamoeba, and this view has been 
followed by Grasse in his Traite de Zoologie, where he groups the 
Flagellata and the Rhizopoda into a subphylum : the Rhizoflagellata. 
To the groups Flagellata and Rhizopoda he gives superclass status 
and groups such as the Dinoflagellata and the Foraminifera are 
termed Classes. 

The Sporozoa were linked by such workers as Doflein (1916) 
with the Flagellata and the Rhizopoda to form the group Plasmo- 
droma. There are certain resemblances between these groups. 
Sporozoans such as Plasmodium have both flagellate sperm and 
amoeboid ookinetes, and spore formation is found in both the 
Flagellata and the Rhizopoda. The Plasmodroma are then 
separated from the Ciliophora with their complex infraciliature. 
Yet even within the Ciliophora there are forms that are possibly 
related to or have something in common with the Flagellata. Thus 
Opalina is according to some writers a ciliate and according to 
others such as Grasse it is a flagellate. 

Such close connexions between the four classes can be inter- 
preted as showing how closely the various groups are related. But 
there is another interpretation. Franz (1924) has suggested that 
the Protozoa are not a strict phylum but instead are a grade of 
organisation. He thinks that there is no good evidence that the 
Protozoa are more primitive than the Metazoa and states that 
the unicellular forms could have been derived many times from 
the Fungi, Algae and the Metazoa. The various groups such as the 
Flagellata, Rhizopoda, Sporozoa or Cilophora would then each be 
polyphyletic and contain animals that have been derived from 
different sources at different times but which are grouped together 
because they have certain convergent morphological characteristics. 

The view that the four classes are polyphyletic is discussed by 
Hyman (1940). " The flagellates themselves appear to be a hetero- 
geneous assembly of groupsthat have probably arisen from a number 
of different sources, possibly bacteria and spirochaetes, many of 
which are provided with flagella. . . . The rhizopods like the 
flagellates constitute an arbitrary assemblage of forms having in 
common the pseudopodial method of locomotion and food capture. 
It is probable that the various orders of rhizopods have arisen inde- 
pendently from the different groups of flagellates, i.e. the class is 



34 



THE PROTOZOA 




Dincclonium 



(A) 




(B). Gymnodinium 

Fig. 7. Relationship between the Protozoa and Algae. The alga 
Dinoclonium has a flagellate spore that resembles a dinoflagellate 
such as Gymnodinium. It is suggested by some authors that 
Gymnodinium is more closely related to Dinoclonium than it is to, 

say, Amoeba. (From Grasse.) 



THE PROTOZOA 35 

polyphyletic. . . . The Sporozoa are again a heterogeneous group of 
which the different orders have probably had separate origins. . . . 
The Ciliata differ so markedly from the other Protozoa in their 
possession of cilia, nuclear dimorphism, and sexual phenomena 
that their relation to them remains problematical." 

So of the four classes of the Protozoa we see that at least three 
are suggested by Hyman as being polyphyletic. 

Baker (1948) has similar doubts about the status of the Protozoa. 
In particular he considers the relationship of the din ofl age 11 ate 
Gymnodinium with the filamentous alga Dinoclonium (Fig. 7). 
During the life cycle of Dinoclonium it develops spores almost 
indistinguishable in structure from Gymnodinium, but Dinoclonium 
is placed in the Algae whilst Gymnodinium and Amoeba are placed 
in the Protozoa. The structure of these spores clearly shows that 
Gymnodinium is more closely related to Dinoclonium than it is to 
Amoeba. Baker concludes that the Protozoa cannot be a mono- 
phyletic group. 

From the evolutionary point of view we therefore have several 
problems in the Protozoa. 

(1) The Protozoa do not seem to be a group of closely related 
animals. It is most likely that they are a polyphyletic group and the 
name " Protozoa " indicates a grade or status rather than a 
natural taxonomic group. In this they would be analogous to the 
group " Vermes " or " Pisces " ; i.e. they show a level of organisa- 
tion and not an evolutionary relationship. (We shall see that this 
problem arises again and again; many of our phyla and classes are 
grades of animals that are not closely related.) 

(2) It is difficult to decide which of the Protozoa are the most 
primitive. The information at our disposal is not sufficient to 
allow us to come to any definite conclusion. 

(3) Each of the four classes probably contains the results of 
convergent development from heterogeneous stocks. 



4— IOE 



CHAPTER 5 



ORIGIN OF THE METAZOA 



When the basic assumptions underlying evolution were dis- 
cussed on p. 13 it was pointed out that if the modern living forms 
were polyphyletic, it should prove difficult to decide their inter- 
relationships and we should have a number of isolated groups of 
animals. This is precisely what we have discovered so far. The 
Viruses, Rickettsiae, Bacteria and Protozoa are all quite distinct 
from one another and their interrelationship is anything but 
clear and certain. We come now to the Metazoa and we have to 
decide whether they can be linked to any of the lower groups of 
animals. 

There are three main views concerning the origin of the 
Metazoa. These are that the Metazoa arose from (1) the colonial 
protozoans, (2) the syncytial protozoans, and (3) the Metaphyta. 

Let us consider each of these views in turn. 

(i) Origin from colonial Protozoa 

Though the Protozoa are often defined as unicellular animals 
there are many protozoans which after division or budding do not 
separate their progeny so that the adult develops a colonial or 
multicellular form. This development into colonies has taken place 
many times within the Protozoa, as can be seen by looking at the 
various classes. The most common examples are found in the 
Flagellata. Simple unicellular forms such as Chlamydomonas can 
at times exhibit an aggregated stage. An example of this is the 
palmella stage during which Chlamydomonas encysts and divides 
asexually ; the results enclosed in a gelatinous case may be regarded 
as a colonial form (Fig. 8). 

In Gonium sociale a group of sixteen Chlamydomonas like 
individuals are associated together in a plate. All these forms are 

36 



ORIGIN OF THE METAZOA 



37 




(A). Chlamydomonas 

Fig. 8. (A) Palmella stage of Chlamy- 
domonas. During this stage the proto- 
zoan divides but the cells remain 
together enclosed in a gelatinous case. 
The stage is " multicellular " though 
it is in fact a resting stage in the life 
history of the protozoan. (FromGrasse 
after Goroshankin.) 




Haematococcus 



Fig. 8. (B) Palmella stage of 

Haematococcus. (From Grasse 

after Wollenweber.) 



alike and at reproduction each of them divides and forms gametes. 
A more complex colony is that of Eudorina in which there are 
sixty-four individuals (Fig. 9). 

Pleodorina illinoiensis and Pleodorina californica show further 
stages in the development of the colony in that a group of cells 
become differentiated from the others and they are unable to take 
part in reproductive activities. There are four somatic cells in 
Pleodorina illinoiensis and thirty-two in Pleodorina californica. 
The soma is even more developed in Volvox, where the majority 
of the cells are unable to take part in the reproductive activity. The 
cells beat their flagella in a co-ordinated manner so that the 
colony can be regarded as having an antero-posterior axis and a 
dorso- ventral axis. The dorsal cells are slightly larger than the 
ventral cells and the colony during locomotion moves slowly 
through the water and does not turn over and over (Fig. 10). 



38 



ORIGIN OF THE METAZOA 




(A). Gonium pectorale 




(B). Eudorina illinoiensis 

Fig. 9. Colonial flagellates. Flagellates such as Gonium and 

Eudorina exist in a colonial form. The colony is active and thus 

differs from the palmella stage shown in Fig. 8. 

(A) From Grasse after Migula. (B) From Grasse after Merton. 



ORIGIN OF THE METAZOA 



39 




(A). Dorsal View 




(B). Side View 



Fig. 10. Colonial flagellates. Volvox though a colonial animal has 
protoplasmic connexions between the units of the colony. In this 
respect it can be regarded as a syncytium. (From Borradaile and 

Potts, after Janet.) 



40 



ORIGIN OF THE METAZOA 



These examples are merely illustrative phases of the develop- 
ment of the colonial habit. There is no evidence that Eudorina 
gave rise to Pleodorina or Volvox. 

Further examples of the development of colonial stages are 
found in the dinoflagellates. Though some forms such as 
Gymnodinium are solitary, others such as Ceratium at times may 
form long chains of individuals joined together in a temporary 
manner. Polykrikos is of interest since it shows a more permanent 
attachment. Polykrikos schwartzi usually contains four nuclei and 
a series of associated sets of flagella. It possesses cnidocysts which 
are manufactured within the cell and are used in catching prey. 
There is also a cytoplasmic connexion between the units of the 
Polykrikos colony. Thus in the related genus Pheopolykrihos 
beauchampi when the animal is touched it contracts up and 
clearly shows that there are four units in interconnexion 
(Fig. 11). 





(A). Normal Animal 



(B). Diagram of shape of animal aftertactilestimulation 



Fig. 11. Colonial dinoflagellates. The dinoflagellate Pheopolykrikos 
is clearly made from four dinoflagellate units. When it is touched it 
changes its form (B) and the four units can be distinguished. (From 

Grasse after Chatton.) 



ORIGIN OF THE METAZOA 41 

Another interesting colonial dinoflagellate is the parasitic form 
Haplozoon. This is found in the gut of polychaetes. It forms 
first of all a small cell which attaches to the gut of the host by 
means of a spike and some filamentous pseudopodia. This cell 
absorbs food from the polychaete and at a later stage divides. 
The results of division do not detach but instead remain in contact 
so that a colonial form of up to several hundred cells is soon 
formed, the number of cells differing from species to species. 
These cells can form a three-dimensional mass with small spaces 
between the cells through which food particles can be transferred 
(Fig. 12). 




Fig. 12. Colonial dinoflagellates. Haplozoon was originally placed 
in the Mesozoa but its spores have typical dinoflagellate structure 
and it is now considered to be a colonial dinoflagellate. (From 

Grasse after Dogiel.) 

The relationship of Haplozoon to the Dinoflagellata is not 
clear at first sight. In fact Haplozoon is so much like a metazoan 
that when it was discovered by Dogiel in 1906 he placed it in the 
Catenata, a new group of the Metazoa. It was not until the work of 
Chatton (1920) that it was shown that the cells at the posterior 
end of Haplozoon detached and developed into four small spores, 
each of which had characteristic dinoflagellate structure. It is for 
this reason that Haplozoon is placed in the Dinoflagellata. This 
reasoning can, if carried to its illogical conclusion, lead one into 
difficulties. Thus Duboscq and Grasse (1933) have shown that 
the mammalian spermatozoan is very much like a protozoan of the 
group Bodoines, yet I doubt if anyone would like to place Man in 
the Protozoa on account of his male gamete ! 

Colonial Protozoa are also found in the other classes. In the 
Ciliphoroa, Anoplophrya forms chains of cells, whilst Carchesium 
and Zoothamnion form branching colonies (Figs. 13 and 14). 
In Zoothamnion the myonemes run throughout the length of the 
colony so that if one part contracts then all the rest contracts. In 
Carchesium the myonemes are restricted to each unit so that they 



42 



ORIGIN OF THE METAZOA 




Fig. 13. Colonial ciliate. Zoothamnion shows a differentiation of its 
parts so that there are feeding zoids and reproducing zoids. In many 
respects it appears similar to a colonial coelenterate. (From Hyman.) 



ORIGIN OF THE METAZOA 



43 



-LI '.':/ I I U. 




Fig. 14. Colonial ciliates. Anoplophrya is colonial only in that the 

cells formed by asexual division often remain attached in the form 

of a chain. (From Borradaile and Potts.) 

contract individually. Zoothamnion is of interest in that it shows 
considerable variation in the structure of its units, the colony 
showing division of labour (Faure-Fremiet (1930); Summers 
(1938)). 

Colonies are found in the cnidosporidian Sporozoa, the spores 
showing well-marked differentiation into cnidocysts each with its 
own nucleus, a spore nucleus and a spore case nucleus (Fig. 15). 
Whether it is justifiable to regard these reproductive units as 




Fig. 15. Colonial sporozoa. The spores of the Cnidosporidia have 
a complex structure, (a) Shows the adult trophozoite and it will 
be seen to resemble the trophozoite of other sporozoans such as 
Monocystis. (b) Spore case together with the undischarged thread 
cells, (c) Spore case with discharged thread cells. 

(a) Sinuolenea. From Grasse after Davis. 

(b) Myxobolus. From Grasse. 

(c) Chloromyxum. From Grasse after Kudo. 



44 ORIGIN OF THE METAZOA 

colonial forms is not clear. Baker (1948) and Ulrich (1950) 
suggest that the Cnidosporidia may be degenerate Metazoa but 
since complex spores are also found in other Sporozoa, e.g. the 
cysts of gregarines, and since the sporozoite of the Cnidosporidia 
is very similar in structure to that of Monocystis, it is more likely 
that they are real sporozoans and not degenerate Metazoa (Fig. 16). 

Under certain conditions the rhizopod Naegleria can aggregate 
so that the cells being joined by a sticky material form a sheet of 
tissue (Willmer 1956). Sphaerozoum is another colonial rhizopod 
— a radiolarian — and in the Mycetozoa there are many forms that 
show colonial structure at certain stages of their life history. What 
is of interest here is that colonies such as Dictyostelium have 
developed a chemical system that keeps the amoebae that go to 
make up the colony in a unit (Bonner 1949). In general the 
Rhizopoda tend to form syncytia more easily than they form 
colonies. On the other hand quite complex multilocular skeletons 
are found in the Foraminifera, but there is usually only one living 
cell present. 

From the foregoing account it is evident that the Metazoa could 
have arisen from the colonial protozoans. There are a large 
number of colonial Protozoa and many of them show differentia- 
tion and division of labour amongst the colony. Whether the 
Metazoa did in fact arise from the colonial protozoans is another 
matter and we must now consider the alternative theories. 

(2) Origin from a syncytial cell 

This theory suggests that the origin of the Metazoa must be 
sought from a protozoan that had many nuclei and which later 
developed membranes separating these nuclei off. The syncytium 
differs from the colony in that the primary unit is the whole 
animal and that it later becomes multicellular. In the colony the 
primary unit is the cell and many of these units come together to 
form the animal. 

The differences between the syncytium and the colony are not 
as clear cut as could be desired. In the main they depend upon 
the absence of cellular boundaries. But what does one say in the 
case of Volvox or Pheopolykrikos where there is protoplasmic 
connexion between the cells (Figs. 10 and 11). Is this a multi- 
cellular animal or a syncytium? 



ORIGIN OF THE METAZOA 



45 










.•••I.C-'-'V ••.'"■ .■' N - 




Fig. 16. Sporozoan structure. Spore cases of gregarines. The 
Cnidosporidea are not the only forms to have complex spore cases ; 
the gregarines have them too. (a) shows the complete spore case, 
(b) shows details of the tube through which the spores are dis- 
charged, (c) shows the tube everted and the spores being discharged 
through it. Such a reproductive spore case with its evertible tubes 
has something in common with the spore case of the Cnidosporidea. 
(a) Gregarina munieri after Schneider, (b and c) Gregarina ovata 
after Schnitzler. (Both from Grasse.) 



46 ORIGIN OF THE METAZOA 

Baker (1948) suggested that since in his definition a cell is "a 
mass of protoplasm largely or completely bounded by a membrane 
and containing within it a single nucleus formed by the telophase 
transformation of haploid or diploid set of anaphase chromosomes," 
that the Ciliophora and the Radiolaria are not cells. They contain 
more than one nucleus and therefore are syncytia. Other syncytia 
are found in the Flagellata (Calonympha, Giardia), Rhizopoda 
(Sappinia, Plasmodiophora), Sporozoa (Myxobolus) and Ciliophora 
(Paramecium). 

The advantage of deriving the metazoan from a syncytial 
protozoan instead of a multicellular one is that in a syncytium such 
as Calonympha or Opalina the animal has an already established 
symmetry and an antero-posterior axis. All it has to do is super- 
impose cell walls on the established pattern. In the development 
of the multicellular form from the colonial pattern one has a series 
of units each with an already established axis and these axes have 
to be amalgamated and altered till the cells form a single unit. 

This view of the syncytial origin of the Metazoa is supported 
by de Beer (1954), who writes, " there are the gravest objections 
to the view that the Metazoa were evolved by aggregation of 
separate protozoan individuals. This may have happened in the 
sponges and, indeed, is the most likely explanation for the lack of 
co-ordination, integration and individuality found in those 
animals. One of the most important features in the acquisition of 
individuality in organisms is axiation and integration throughout 
the body. The only way in which this can be imagined as having 
occurred in the transition from Protozoa to Metazoa is by means 
of internal subdivision of the protozoan cell, by cellularisation. 
Nor is it difficult to imagine how this might have been brought 
about, since there are Protozoa such as the Ciliate Infusoria, 
Haplozoa and some Sporozoa which possess many nuclei, and it 
would only be necessary to separate these by cell walls in order to 
obtain the required organisation for the primitive Metazoa." 

One difficulty comes when we consider whether the syncytium 
has any inner cell walls or not. Thus if it can have cell walls 
which do not divide the parts completely, then animals such as 
Volvox which has connexions between the adjacent cells are 
syncytia (Fig. 10). It is true that it is possible to consider that 
Volvox has arisen by the accumulation of Chlamydomonas like 



ORIGIN OF THE METAZOA 47 

individuals whilst it is not possible to find any sub-unit for 
Opali?ia. Nevertheless the differences between syncytia and 
colonies are not as clear cut as has sometimes been supposed. 

Another difficulty arises when we consider the stage during the 
life cycle during which the protozoan is syncytial or colonial. 
Thus many of the Protozoa form spores during reproduction and 
these may be localised in spore cases. Are these to be regarded as a 
multicellular stage? If so, then the palmella stage of Chlamy- 
domonas could be a colonial form (Fig. 8). The Cnidosporidia 
have many nuclei only during the reproductive (spore-forming) 
stage; their trophozoite is unicellular and only has one nucleus. 
Equally well the ciliate Anoplophrya, which does not separate 
its asexually produced cells from the parent immediately they are 
produced, can be considered as a colonial form (Fig. 14). In 
effect the situation is quite difficult to resolve and depends to a 
large extent on the relative duration of the multicellular stage and 
the part that it plays in the life of the animal. Hadzi (1953) 
has suggested that one of the major differences between the 
Protozoa and the Metazoa is that the Protozoa have their major 
phase in the reproductive stage whilst the Metazoa have their 
major phase in the vegetative stage. 

(3) Origin from the Metaphyta 

The third view concerning the origin of the Metazoa is that 
they arose from the plants, the Metaphyta. It has already been 
mentioned that Franz (1926) thought that the Protozoa were in 
fact derived from the Metaphyta and the Metazoa. Baker (1948) 
suggested that the Metazoa arose from plant-like protozoans. 
" The unicellular plant absorbs nutriment from all sides equally, 
and when in the course of ontogeny or phylogeny it becomes a 
metaphyte there is no fundamental change in this respect; a cell 
divides without separation and the two products continue to 
absorb nutriment over most of their surface. The passage from 
unicellular form to the metaphyte is therefore easy. In the case of 
animals, however, there is an important change when a unicellular 
form becomes a metazoon; a new method of feeding must be 
adopted. . . . The difficulties would be greatest when the pro- 
tozoon had a localised mouth. If the products of such an animal 
were to adhere together and each were to acquire its own mouth, 



48 ORIGIN OF THE METAZOA 

no advance could be made to the evolution of a metazoan alimen- 
tary canal. This suggests that the Metazoa may have arisen 
from primitive Protozoa unprovided with localised organs of 
assimilation." 

Hardy (1953), following on from Baker's argument, suggests 
that " the Metazoa have not been derived from the Protozoa at all 
but from relatively simple metaphytes, which after they had 
evolved from the protophytes began, perhaps as a result of a 
shortage of phosphates or nitrates, to capture and feed on small 
organisms as do the higher insectivorous plants." 

This is an interesting suggestion but one that can be criticised 
on several grounds. There is no evidence that metaphytes such as 
the Algae can withstand food shortages by catching animacules. 
The thick cellulose cell wall around the metaphytes, though a 
protection, would also tend to prevent them from developing 
pse dopodia rapidly enough to catch protozoans. The insectivor- 
ous plants, it will be remembered, all develop special insect- 
catching mechanisms, and even so they still retain their photo- 
synthetic ability. 

Baker's arguments in favour of the origin of Metazoa from 
plant-like protozoans can also be contested. There is no reason 
why a metazoan type of alimentary canal should develop in the 
first stages of the evolution of the Metazoa. The " alimentary 
canal " of the sponges is not really comparable in function to that 
of the higher metazoans, and even in the coelenterates there is a 
considerable amount of amoeboid activity in the gut cavity. What 
would appear to be more important than the development of an 
alimentary canal is that the cells of the body should have some 
continuity and interconnexion with each other so that food 
material can be passed easily, from one cell to the other. Such a 
process probably does occur in the colonial ciliates such as 
Zoothamnion which have well-developed gullets. Summers (1938) 
suggested that food material was probably passed along the stalk 
of Zoothamnion. Here then we have the case of a protozoan with a 
well-defined mouth forming a colony. Furthermore this colony 
shows differentiation and division of labour, some of the polyps 
being more intensive feeders than others. 

There is no reason why a protozoan with a definite polarity 
should not lose this polarity and develop into a colonial form. 



ORIGIN OF THE METAZOA 



49 



Willmer (1956) in discussing the change of form of Naegleria 
points out that the rhizopod can be in any one of three phases: 

(1) in the flagellate stage with a definite polarity; 

(2) in the amoeboid phase with pseudopodia coming from all 
over the body ; 

(3) aggregated in the form of a sheet of tissues. 

It would seem that this protozoan has no difficulty in losing its 
polarity and therefore that the difficulties raised by Baker con- 
cerning the changes from Protozoa to Metazoa are not as great 
as he suggests. 

What conclusion then can be drawn concerning the possible 
relationship between the Protozoa and the Metazoa? The only 
thing that is certain is that at present we do not know this relation- 
ship. Almost every possible (as well as many impossible) relation- 
ship has been suggested, but the information available to us is 
insufficient to allow us to come to any scientific conclusion regard- 
ing the relationship. We can, if we like, believe that one or other 
of the various theories is the more correct but we have no real 
evidence. 




CHAPTER 6 



THE MOST PRIMITIVE METAZOA 



We have seen so far that the Metazoa can be derived either from 
syncytial protozoans, from multicellular protozoan colonies or 
from the Protophyta. To some extent the theory that one chooses 
as the most probable will depend upon which group is considered 
to be the most primitive of the Metazoa. Thus if one considers the 
Sponges as the most primitive of the Metazoa then one could 
suggest a link between the Protozoa and the Metazoa via the 
Choanoflagellata. If on the other hand one thinks that the 
Acoelous Platyhelminthes are the most primitive metazoans, then 
one could consider that the link with the Protozoa was via the 
complex ciliates. It is therefore important to decide which are the 
most primitive of the Metazoa, but before this can be done one 
has to consider four questions. 

(1) Which of the metazoan groups can be considered the 
earliest to have evolved? 

(2) Which are morphologically the most simple of the Metazoa? 
(This will not necessarily be the first group to have evolved.) 

(3) What is the relationship between the major groups of the 
lower Metazoa? 

(4) Can the Metazoa be considered as a polyphyletic group 
with more than one origin from the simpler living forms? 

The metazoans that will be considered here are the five groups : 

(1) Porifera. 

(2) Mesozoa. 

(3) Coelenterata. 

(4) Ctenophora. 

(5) Platyhelminthes. 

Before discussing these questions it will be as well to indicate 

50 



THE MOST PRIMITIVE METAZOA 51 

the various uses of the expressions " primitive; simple; advanced; 
radial symmetry and bilateral symmetry," since these terms will 
frequently be used in the following discussion. 

Primitive and simple 

There are two terms that must be distinguished and used 
carefully. The first term is " simple." If an animal has a 
morphological structure made up from a few basic units, then such 
an animal can be regarded as having a simple structure. Other 
animals may have many different units arranged in a variety of 
patterns ; they can then be regarded as having a complex structure. 
These two groups, the simple and the complex, can also be 
described as having a low level of complexity or a high level of 
complexity. 

The second term is " primitive." This means that of two 
structures or conditions, one arose some time before the other. The 
concept of "time of origin" is the critical point in determining 
whether a structure is primitive or not. Because an animal has a 
simple morphological pattern it does not mean that it had an 
early evolutionary origin and therefore is in a primitive condition. 

It is perhaps unfortunate that during most courses of Zoology 
the students are taken from the Protozoa to the Primates and 
shown the way in which the complexity of structure increases. 
Quite often the student becomes puzzled when he deals with the 
Mollusca. Should they come before the annelids, between the 
annelids and the arthropods, or after the arthropods? It is clear 
that this problem confuses two issues: firstly the complexity of 
molluscs in relation to that of the annelids and the arthropods, 
and secondly the time of origin of the molluscs, i.e. did they arise 
before or after the annelids? 

The student is usually taught that certain conditions can lead 
to a simplification of morphological form and that clues other than 
purely morphological ones must be used to elucidate an animal's 
phylogenetic position. In particular this holds when we come to 
deal with parasitic animals. Thus the larval form of Sacculina 
quite clearly shows the crustacean ancestry of the parasite even 
though the morphology of the adult is not at all typical of the 
Crustacea. Though the parasitic habit is usually associated with 
certain morphological changes, the other specialised ecological 

5— IOE 



52 THE MOST PRIMITIVE METAZOA 

conditions are not often given equal credit for determining and 
shaping an animal. Thus various morphological conditions will be 
associated with a pelagic life, with burrowing, with living in sand, 
with being a very large animal or being a very small animal. All 
of these tend to alter the morphology of the animal and make it a 
successful living animal, not just a representative of a hypothetical 
idea; that of, say, a crustacean. An example of such an environ- 
mental effect can be seen when we come to consider which is the 
more primitive, radial symmetry or bilateral symmetry? (Fig. 17.) 

Radial symmetry 

This type of symmetry is often found in sessile or pelagic 
animals. They usually have an oral and an aboral surface but 
otherwise any diameter cut at right angles to the oral-aboral axis 
should divide the animals into tw r o similar halves. In fact most of 
the animals that are radially symmetrical do not fit in with this 
definition since they usually have some irregularity in their 
organisation, i.e. mesenteries, tentacles, madreporite, which allow 
only certain sections at right angles to the oral— aboral axis to 
divide the animal into equal halves. 

Bilateral symmetry 

The animals that show bilateral symmetry are organised into 
an antero-posterior axis and a dorso-ventral axis. In addition there 
is one plane and one plane only that will separate the animals into 
equal right and left halves. A radially symmetrical animal will 
have many such planes. Most of the Metazoa that are not pelagic 
or sessile show a bilateral symmetry. (Fig. 17.) 

One is often taught that the coelenterates and the echinoderms 
show a basic radial symmetry and that the other Metazoa are 
bilaterally symmetrical. Since the echinoderms and coelenterates 
are sometimes placed at the foot of the metazoan evolutionary 
tree it is not difficult to associate radial symmetry with a primitive 
habit and to assume that the bilaterally symmetrical condition is 
the more advanced. On the other hand, it is equally true that 
radial symmetry is found in sessile or floating animals whilst 
bilateral symmetry is found in crawling or swimming animals. 
We are therefore left with the question, "To what extent does the 
symmetry of an animal indicate its primitiveness and to what 
extent does it reflect the habits of that animal? " 



THE MOST PRIMITIVE METAZOA 



53 




(A). Radial Symmetry 




(B). Biradial Symmetry 




(C). Bilateral Symmetry 



Fig. 17. Types of symmetry. 



54 THE MOST PRIMITIVE METAZOA 

There are certain exceptions to the generalisation that sessile 
animals are radially symmetrical. Thus, as previously mentioned, 
even amongst the coelenterates there are many planes that will not 
divide the animal into two equal halves, this being due to the 
development of tentacles, gonads, batteries of nematocysts, 
mesenteries and siphonoglyphs. In other Metazoa it is rare to find 
radial symmetry. Thus in the Rotifera, neither Trochosphaera 
nor Melicerta are perfectly radially symmetrical. In the Annelida 
Sabella is not radially symmetrical; its parapodia still show a 
bilateral symmetry. In the barnacles, though there is some 
tendency towards a radial symmetry as illustrated by the skeletal 
plates, the internal symmetry of other organs such as the legs, 
digestive system and nervous system is a bilateral one. Other 
sessile animals such as the Crinoids, Ascidians and Pterobranchi- 
ates do not show perfect radial symmetry. On the other hand the 
ctenophores that take up a crawling habit such as Coeloplana and 
Ctenoplana do show a very interesting bilateral (biradial) symmetry. 

These examples indicate that subject to certain basic limita- 
tions, the life that an animal leads will influence its shape and 
basic symmetry. The question, " Is radial symmetry more 
primitive than bilateral symmetry? " should perhaps be more 
correctly replaced by the question, " Is the sessile or pelagic habit 
more primitive than the swimming and crawling habit? " The 
answer to the latter question is at present unknown. 

(1) The Sponges (Porifera) 

The sponges are peculiar multicellular animals with an organisa- 
tion quite different from that of the other Metazoa. They have a 
skeletal system and three layers of cells, pinacocytes, amoebocytes 
and choanocytes, but they have no organ systems such as an 
excretory or a nervous system. They have a very simple digestive 
system in which there is no real mouth or gut. The sea water 
around the animal passes through a series of apertures into the 
centre of the sponge and in doing so is filtered, the food being 
taken up by the choanocytes and the amoebocytes. 

The organisation of the sponge is very simple in that a sponge 
can be passed through the meshei of a net and so separated into its 
individual cells. These cells can later aggregate and form an 
organised sponge with the cells in their correct relative position; 



THE MOST PRIMITIVE METAZOA 55 

the mechanism of this interesting rearrangement is not yet under- 
stood. This type of cellular organisation is found in the higher 
Metazoa where it has superimposed on it the co-ordinating 
influence of a nervous and hormonic integration, both of which 
are apparently absent in the sponges. The high degree of skeletal 
material relative to the small amount of living protoplasm makes 
the sponges very poor food and so a relatively successful group of 
animals. 

Are the sponges a primitive or an advanced group of animals? 
To answer this question we should have to know the time of 
origin of the sponges and there is no certain information on this 
point. Instead we can examine the apparently simple characters 
and the apparently complex characters and attempt to derive some 
satisfaction from this. The reader should always be on his guard 
when consulting such lists ; it is not possible to come to a conclusion 
merely by seeing which of the two lists is the longer ! 

Simple characteristics 

(1) The layers of the body are loosely organised. 

(2) The layers of the body do not correspond to the ectoderm, 
mesoderm and endoderm of the higher forms. 

(3) There is no definite body form. 

(4) They have choanoflagellate cells like those present in the 
choanoflagellate protozoans. 

(5) There is no nervous system. 

(6) There is no excretory system. 

(7) There is no mouth. 

(8) The gut (gastral cavity) shows little differentiation. 

(9) They have a high regenerative capacity. 

(10) They have a well-developed system of asexual reproduction. 

(11) The larvae have well-developed flagella. 

Complex characteristics 

(1) They have three layers of cells (some coelenterates have only 
two layers of cells). 

(2) They have a well- developed middle layer, the " mesen- 
chyme," with an elaborate skeletal system. 

(3) They have some differentiation within the layers, e.g. the 
pore cells. 



56 THE MOST PRIMITIVE METAZOA 

(4) They have a gut. (The Mesozoa have no gut.) 

(5) The gemmules with which some sponges carry out asexual 
reproduction are quite complex in structure. 

(6) They have eggs and sperm. 

(7) The embryo gastrulates in a complex manner, by ingression, 
epiboly and delamination. 

(8) They have a well-developed amphiblastula larva. 

(9) The blastopore is aboral, in most other metazoans it is oral. 

(10) An inversion of the layers occurs during embryology. 

(11) The sponges are the only animals to have the main body 
aperture the exhalant one. 

Various excuses can be made for each and every one of the simple 
or specialised characteristics. Thus, to deal with a few of them : 

(1) Though it is correct that the layers are loosely organised this 
may be due to the fact that the sponges do not depend on the 
hydraulic pressure of the gastro-vascular cavity to maintain their 
shape. They have a well-developed skeletal system that takes care 
of this. The animal remains intact even though there are many 
series of canals running through the body, and the cells are not 
firmly cemented together. 

(2) The layers do not correspond to the ectoderm, endoderm 
and mesoderm of the higher animals and there is no evidence that 
at one time they did correspond. There is not one piece of evidence 
to show that the normal triploblastic condition evolved from that 
shown by the sponges. 

(3) Many sponges such as Euplectella and Poterion do have a 
definite shape. 

(4) Choanoflagellate-like cells are also found in the endoderm 
of some coelenterates, annelids and molluscs. Electron microscope 
studies show that the collar is a series of protoplasmic filaments 
that project out of the cell in much the same way that the digestive 
filaments project from an endodermal cell (Rasmont et al. 1958). 

(5) There are other animals that have no nervous system, e.g. 
the Mesozoa. 

(6) The Mesozoa, Coelenterata, Ctenophora and Acoela have 
no specialised excretory system. 

(7) The gut in the sponges though it shows little differentiation is 
at least a gut. There is no gut in the Mesozoa, Acoela or Pogonophora. 



THE MOST PRIMITIVE METAZOA 57 

(8) A high regenerative capacity does not indicate that an 
animal is simple. Thus in the coelenterates, the polyps have good 
regenerative capacity, the medusae do not. In the annelids the 
Oligochaeta have good regenerative capacity, the Hirudinea do 
not. In the amphibians the Urodeles regenerate well, the Anura 
do not. 

(9) There is no evidence that asexual reproduction is necessarily 
more primitive than sexual reproduction. 

It can be seen that the situation is not as straightforward as the 
lists might at first sight make it appear. It should also be re- 
membered that though one might believe that the sponges are 
more simple than the coelenterates from the point of view of their 
morphology and life cycles, this is no reason for thinking that they 
necessarily developed some time before the coelenterates. The 
sponges may have been a happy afterthought of the Protozoa 
after they had given rise to the coelenterates ! 

Which group of animals did the sponges come from? There 
are three different answers to this question. The first derives 
the sponges from Protozoa such as the choanoflagellates or 
Volvocinae. The second derives the sponges from a Gastrea type 
of animal which gave rise to both the coelenterates and the 
sponges. The gastrula probably came from a colonial protozoan 
such as Volvox. The third answer derives the sponges from a 
coelenterate source. Each of these views has something in its 
favour and something against it. 

Origin from the Protozoa 
(i) From the Choanoflagellata 

The inner layer of cells in the sponges is composed mainly of 
Choanocytes. These in many ways resemble the cells of the 
choanoflagellate protozoa and on this basis it has been suggested 
that the sponges might be derived from these protozoans. 

In 1880 Saville Kent described a colonial choanoflagellate 
called Proterospongia (Fig. 18). This consisted of a flat plate of 
about forty cells. The other cells had the normal choanoflagellate 
structure whilst the inner cells were amoeboid. Periodically a 
choanoflagellate cell would withdraw its flagellum and become 
amoeboid, whilst an amoeboid cell would take up the position and 



58 



THE MOST PRIMITIVE METAZOA 




(A). Proterospongia 




(B). Sphaeroeca 

Fig. 18. Colonial protozoa that in some ways resemble sponges. 

(A) Proterospongia. (After Saville Kent.) This is now believed 

to be a fragment of a fresh- water sponge. 

(B) Sphaeroeca; single choanoflagellate protozoan and the colonial 

form. (From Grasse after Lauterborn.) 



THE MOST PRIMITIVE METAZOA 59 

structure of a choanoflagellate cell. This protozoan had in many 
ways the structure that one might expect to find in an ancestral 
sponge and it has usually been figured as such in textbooks of 
zoology. 

Proterospongia is not a common protozoan. Recently Tuzet 
(1945) has investigated the antecedents and morphology of this 
protozoan and she decided that in fact Saville Kent was the only 
person ever to have definitely seen Proterospongia and that what he 
saw was not a protozoan but a small fragment of an actual sponge. 
Tuzet concludes, " Pour nous. ... La Proterospongia de Saville 
Kent n'est pas autre qu'un corps de restitution d'Eponge d'eau 
douce." Proterospongia is nothing more than a restitution body 
of a fresh- water sponge. If this is true it is not surprising that 
Proterospongia has many sponge qualities. On the other hand 
Grondtved (1956) has described a new species of Proterospongia, 
P. dybsoensis, in which there are three to ten cells arranged in a 
linear row per colony. These colonies were often found in very 
large numbers, up to 2,300 colonies per litre of water, and 
Grondtved thought that they might be fragments from a larger 
colony except for the fact that they were all so much alike. There 
does seem to be quite a considerable difference between the row of 
three to ten choanoflagellate cells embedded in a gelatinous 
common envelope and Proterospongia as described by Saville Kent, 
where the choanoflagellate cells migrated into the interior of the 
colony and took up amoeboid structure. For this reason it is not 
clear whether the new species rightly can be placed in the genus 
Proterospongia . 

(2) From the Volvocinae 

There are certain resemblances between the embryonic develop- 
ment of Volvox and that of certain sponges. Duboscq and Tuzet 
(1937) showed that in Grantia the embryo developed inside a 
membrane. The blastula is made up of two types of cells, 
flagellate ones and non-flagellate ones. In the blastula all the 
flagellate cells point inwards at first but during the course of 
development the blastula turns inside out so that the flagella now 
point outwards. This phenomenon is called inversion and is 
shown diagrammatically in Fig. 19. The larva is then liberated 
as an amphiblastula larva with the flagella at one end. The other 



60 



THE MOST PRIMITIVE METAZOA 






Fig. 19. Diagram to show inversion during the embryology of the 

sponge Grantia. The embryo develops at first with its flagella 

pointing inwards (1). The embryo slowly inverts (2-3) so that its 

flagella now point outwards. (After Duboscq and Tuzet.) 



end of the larva has the non-flagellated cells and after swimming 
for some time the amphiblastula larva settles on the substratum, 
the non-flagellated cells grow over the flagellated cells so that the 
animal takes up the structure of an adult sponge. 

A similar inversion takes place during the development of the 
daughter colonies of Volvox (Pocock 1933). During the asexual 
development of a daughter colony the daughter cells divide and 
form a sheet of cells. These cells are orientated in the same manner 
as the parent cells, the flagella pointing outwards, but as division 
proceeds the daughter cells form a ball with the flagella pointing 
towards the centre of the ball. The colony is not a complete 
ball since there is a hole at the top. The colony now proceeds to 
turn itself inside out through this hole in much the same way as 
one might push a tennis ball inside out through a hole in the 
wall (Fig. 20). This results in a colony with the flagella all 
pointing outwards, the inversion taking some two hours. 



THE MOST PRIMITIVE METAZOA 



61 




Fig. 20. Diagram to show inversion during the development of 
daughter cells in Volvox. The flagella of the daughter colony all 
point towards the inside of the colony. Inversion takes place and the 
colony turns inside out so the flagella now point outwards. (After 

Zimmerman.) 



In both Volvox and Grantia the cells develop in the same initial 
relationship to the embryo as they do to the parent layers. Hyman 
(1940) was impressed by the similarity between the inversion in 
Volvox and the sponges and suggested that this might indicate a 
common ancestry. On the other hand Tuzet (1945) considered 
the situation as one of convergence which does not indicate any 
underlying phylogenetic relationship. 



62 the most primitive metazoa 

Origin from the Gastrula 

This view holds that the sponges evolved from some form of 
gastrula and was propounded mainly by Ernst Haeckel. Haeckel 
from his studies of the embryology of the sponges (1872) decided 
that the larval form of the sponge was a gastrula larva. He also 
thought that certain other animals such as Haliphysema were 
primitive sponges, though he later changed his mind. 

Haeckel's views have had considerable influence on our current 
zoological concepts. It was Haeckel who devised many of our 
current words such as Phylum, Blastula, Morula, Gastrula, 
Ontogeny, Phylogeny and many more. It is perhaps relevant that 
we should spend a little time reporting how Haeckel developed his 
ideas and concepts and the way in which these fitted in with 
views on the origin of the sponges. 

Haeckel spent many years developing his views on the phylogeny 
of the animal kingdom. In effect he studied both the structure 
and embryology of each of the various groups. Then by marrying 
the facts and ideas from comparative anatomy and embryology, 
Haeckel developed a sweeping plan of the relationship and evolu- 
tion of various animal groups. He supported his schemes with 
detailed arguments and when his opponents failed to understand 
his arguments Haeckel devastated them with a barrage of crushing 
sarcasm against their misinterpretation of his own specialised 
terminology. 

Simply stated, Haeckel's view was that the most primitive 
animal was a small non-nucleated mass called the Monerula (Fig. 
21). This was followed by a later group of animals that had a 
nucleus and this state was called the Cytula. The Protozoa are 
at the cytula stage. The next group of animals formed a solid 
mass of nucleated cells called the Morula. The Morula led to a 
more complex form that had a hollow centre and a single layer of 
cells, the Blastula. Certain cells such as Volvox are almost at the 
Blastula stage. The next stage in evolution was the Gastrula stage, 
in which the animal had a double wall, a ciliated exterior and a 
hollow gut. 

The Gastrula assumed tremendous importance in Haeckel's 
phylogenetic speculations. He thought that the Gastrula was the 
ancestor of all the Metazoa, that it occurred in all the Metazoa at 



THE MOST PRIMITIVE METAZOA 



63 






(o) Monerula 



(b) Cytula 



(c) Morula 





(d) Blastuta 



(e) Gostrula 



Fig. 21. Haeckel's concept of levels of organisation. He suggested 
that animals evolved through the successive adult stages shown in the 

above figure. 



some stage of their embryonic development and that a group of 
animals existed which were adults but which were still at the 
gastrula stage. Such animals were not known, but later Haeckel 
thought he discovered such an adult group of animals and 
he called them the Physemaria, an example of which was 
Haliphysema. 

Haliphysema, according to Haeckel, had the structure of a little 
vase. The walls of this vase were made up of two layers of cells ; 
the inner layer was flagellated, the cells having a collar-like 
structure. The outer layer was made up from a syncytium of 
cells. These outer cells took up stones and spines from the environ- 
ment and covered the animal with a protective layer. They were 
adults, since Haeckel described a series of gonadial cells which 
were formed from the endoderm and then became liberated into 



64 



THE MOST PRIMITIVE METAZOA 



the gut (Fig. 22). Many genera related to Haliphysema were dis- 
covered and they all showed certain resemblances to sponges, i.e. 
a central cavity lined with flagellated cells, and an outer layer of 
cells covered by a skeleton which in this case differed from that 
of a sponge in that it was not secreted but picked up from the 
substratum. These animals assumed such tremendous importance 
to Haeckel that at one time he derived all the Metazoa from the 
Physemaria, but at a later date he decided that the Physemaria 
were on a side line from the main Gastrula. 





(b) Transverse section 




(a) Longitudinal view 



(c) Isolated choanoflagellate cell 



Fig. 22. Haeckel's view of the structure of Haliphysema. He 
thought that the structure was that of a simple sponge, (a) shows 
the whole animal with part of the body cut away to demonstrate 
the inner structure, (b) is a transverse section of Haliphysema and 
shows the gonads migrating into the interior of the animal, (c) shows 
in more detail the structure of the inner flagellate cells. It is 
doubtful if such a detailed pattern exists in Haliphysema or if such a 
simple condition exists in any sponge. 



THE MOST PRIMITIVE METAZOA 65 

A difference of opinion arose between Haeckel and other 
workers over the structure of the Physemaria. Saville Kent in 1878 
from studies of both living and fixed material decided that 
Haliphysema was no sponge but instead a foraminiferan rather 
like Euglypha. He was unable to see any of the internal details 
described by Haeckel. A controversy soon arose between Haeckel 
and Saville Kent and it was left to Ray Lankester, as a friend of 
Haeckel, to enter the controversy in the role of adjudicator. 

Lankester asked Saville Kent for specimens of Haliphysema 
and then examined them both alive and in the fixed and stained 
condition. After considerable examination Lankester (1879) 
decided that Saville Kent was perfectly correct in his assertions 
and that the specimens were clearly those of a foraminiferan. But 
the matter did not end there. Lankester ingenuously decided that 
the answer to such a controversy was extremely simple. He sug- 
gested there must be two different genera of animals which from 
the outside looked exactly alike but one of these had been studied 
by Professor Haeckel whilst the other had been studied by Mr. 
Saville Kent. Lankester had no doubt that the isomorph studied 
by Haeckel would have the structures that Haeckel had described 
and he hoped that Professor Haeckel would supply him with some 
specimens. It does not appear that such specimens were ever 
sent to Lankester. 

Perhaps something should be said in Haeckel's defence. In a 
recent paper on Haliphysema tumanowiczii, Hedley (1958) describes 
the way in which the protozoan often picks up sponge spicules 
and covers itself with these. Also certain individuals were 
multinucleate, a condition which in certain circumstances might 
be confused with some of the conditions described by Haeckel. 

Haeckel continued to believe in the importance of the Physemaria 
though he thought that the Gastrula was more important (1899). 
He derived all the Metazoa from the Gastrula and stated, " I 
regard the Gastrula as the most significant and important 
embryonic form in the whole animal kingdom. It occurs amongst 
the sponges, Acalephe, the Annelida, Echinodermata, Arthropoda, 
Mollusca, and the Vertebrata as represented by Amphioxus. In 
all these representatives of the most various animal stocks, from 
the sponges to the vertebrates, I deduce, in accordance with the 
Fundamental Biogenetic Law, a common descent of the whole 



66 THE MOST PRIMITIVE METAZOA 

animal world from a single unknown stock form, Gastrula or 
Archigastrula, which was essentially like the gastrula." (Mono- 
graph on Calcispongiae.) 

These ideas were not accepted even in Haeckel's own time. 
Thus both Claus and Schmitt disagreed with him, and as Radl 
(1930) states, " The popular idea of the method of the scientist is 
that he assembles a series of definite facts upon which he founds his 
case. We see that this is not always the case. It is not true that the 
facts which told against the Gastrula were unknown at the time 
when the theory was propounded, or that the theory was gradually 
discredited as the facts which contradicted it were gradually 
accumulated until it finally had to be abandoned. Everything that 
has ever been cited against the theory was known when the theory 
was put forward; nevertheless it was widely accepted. Today 
some still accept it, others do not." Though it is perhaps an over- 
statement that " everything that has ever been cited against the 
theory was known when the theory was put forward," Radl's 
point is made quite clear. The theory was often accepted because 
it was attractive and not because it was supported by detailed 
verified factual information. 

One may conclude, therefore, that there is at present no 
evidence that the sponges arose from an adult gastrula. They have 
neither a hollow gastrula larva nor are there any simple sponges 
that are still in the gastrula condition during the adult stage. The 
situation as Haeckel saw it was based on over-simplification and 
misinterpretation of the evidence. 

A modification of the Gastrula theory has recently been pro- 
posed by Jagersten (1955). He suggested that the primitive 
blastula gave up living and swimming in the sea and started to 
crawl on the sea bottom (Fig. 23). It modified its structure to 
become a " Bilateroblastea " with a flattened ventral surface, an 
arched back, a few sensory cells at the front of the body and the 
sexual cells inside the body. The centre of the body was hollow 
and not filled with mesodermal cells. At first the animal fed 
phagocytically all over the body surface, but as food particles 
accumulated on the ventral surface the phagocytic ability became 
restricted to the ventral region. This then became raised from the 
ground till the animal took up the shape of the " Bilaterogastrea ' 
as shown in Fig. 23. 



THE MOST PRIMITIVE METAZOA 



67 





(b) 




Fig. 23. Bilaterogastrea theory. Jagersten has suggested that 
the primitive larval form was that of a bilaterogastrea. The 
planula larva (a) settled on the ground, raised its ventral surface 
to accommodate food material (b, c) and so developed a gut (d, e, 
f). It then takes up the form of a Bilaterogastrea. (From 

Jagersten.) 



Jagersten's scheme is derived from his view that the primitive 
form had a hollow centre and was not filled with mesenchyme. 
The reason he thinks that the primitive form had a hollow interior 
is that otherwise the various groups of animals such as the 
coelenterates, platyhelminthes, etc., would have had to develop a 
hollow gut independently and on several different occasions. 
Secondly, though he agrees with the statements that the endoderm 



6— IOE 



68 



THE MOST PRIMITIVE METAZOA 



in the Hydrozoa is formed by the wandering in of cells, in the 
Scyphozoa, Anthozoa and higher animals the endoderm is often 
formed by invagination. Since on other grounds Jagersten thinks 
that the Anthozoa are more primitive than the Hydrozoa, he 
suggests that invagination is the more primitive system in the 
formation of endoderm and that the primitive larva had a hollow 
centre, i.e. a gut, and was not solid as has been suggested by 
Hyman (1940). 

The following system is therefore suggested by Jagersten to 
explain the development of the Porifera from the bilaterogastrea. 
The bilaterogastrea settled on the sea floor and placed the 
middle of its elongated mouth on the substratum. The water and 
food material flowed in through the mouth and out via the anus. 
The mouth later became folded and developed a series of pores as 
shown in Fig. 24. The anus remained a single structure and 
migrated to a dorsal position to become the exhalant opening. The 
animal then had the form of a sponge though it would still have 
to develop the peculiar histological structure of the Porifera. 
J agersten's view is of interest in showing what could have happened. 
Whether the Porifera did actually arise in this manner is open to 
doubt. 




Fig. 24. Jagersten's view of the evolution of the sponges. The 
bilaterogastrea settled on the bottom (a), raised its mouth from 
the substratum (b) and then divided the mouth into many oscula 
(c). The anus then migrated dorsally (f, g). (e) is a transverse 

longitudinal section of a. 



the most primitive metazoa 69 

Origin from the Coelenterata 

This view, that the coelenterates and the Porifera have close 
ancestral affinities is quite an old one. It is based on the fact that 
the coelenterates and the sponges often have a solid planula-type 
larva during their embryology. Lankester (1890) strongly sup- 
ported this view when he emphasised the differences between the 
planula larva and the Gastrula. He thought that there was no 
indication that the sponges ever had a gastrula stage but that 
instead the resemblance was in the solid blastula. 

In Leucosolenia the fertilised egg divides to form a sixteen- 
celled hollow blastula. The majority of the cells are flagellated but 
a few at one end are non-flagellated. These non-flagellated cells 
together with a few of the flagellated cells migrate into the interior 
of the blastula and fill the central cavity. The result is a solid 
blastula. This settles on the ground, flattens and the inner cells 
then migrate out on top of the flagellated cells. They then become 
the pinacocytes and amoebocytes, whilst the flattened flagellated 
cells turn into the choanocytes. 

Though there are certain similarities between the development 
of the sponges and the coelenterates, it is difficult to know how 
much reliance can be placed on them. Thus Balfour (1880) was 
quite clear that there was no relationship between the cell layers 
of the sponges and those of the coelenterates since the inner layers 
of the sponge embryos come to lie on the outside of the adult. 
For the same reason Delage (1898) suggested that the sponges have 
their endoderm on the outside and the ectoderm on the inside and 
that the sponges should be called the " Enantiozoa " for this 
reason. It is of interest that Saville Kent (1880) described the 
way in which the outer cells of the sponge, the pinacocytes, could 
take in food particles. 

It is possible to enumerate the similarities and differences 
between the sponges and the coelenterates as follows. 

Similarities between sponges and coelenterates 

(1) They are both aquatic and free-living animal groups. 

(2) They have spicules in their skeleton, which are either 
calcareous or horny. 

(3) They have flagella. 



70 THE MOST PRIMITIVE METAZOA 

(4) Amoebocytes are present in both groups. 

(5) The main body cavity is neither a haemocoel nor a coelom. 

(6) There are no excretory organs. 

(7) They occasionally have mesenchyme but never mesoderm. 

(8) They form buds or gemmules for asexual reproduction. 

(9) They have sexual reproduction with sperm and eggs. 

(10) They both form colonies. 

(11) They both have a high regenerative capacity. 

(12) They have a solid blastula larva (stereoblastula) before the 
amphiblastula or planula larva develops. 

(13) The anterior end of the larva becomes attached to the 
ground. 

(14) The sex cells are formed by interstitial cells or amoebocytes. 

Differences between sponges and coelenterates 

(1) The sponges have many entrances to the body cavity. 

(2) The main body opening of the sponges is the exhalant one. 

(3) The sponges have choanocytes (but see p. 56). 

(4) The outer layer of the sponge is never ciliated in the adult. 

(5) There are no nematocysts in sponges. 

(6) The sponges have no muscles or musculo-epithelial cells 
(except porocytes). 

(7) The sponges have no nervous system or sense organs. 

(8) The sponges do not show polymorphism. 

(9) The sponge spermatozoa are sometimes carried to the egg by 
amoebocytes. 

(10) The sponges never form a compact skeleton such as is seen 
in certain coelenterates such as the corals. 

(11) The adult sponges are never pelagic. 

(12) There is no clear homology between the layers of the 
sponges and the layers of the coelenterates. 

Though the above lists do not actually prove anything they 
do indicate that the differences between the coelenterates and the 
sponges are quite considerable and basic. It is thus doubtful if 
there is any close relationship between these two groups. It is 
also impossible to state whether the sponges arose earlier than the 
coelenterates. Their organisation is less complex in some ways, 
but this again does not necessarily mean that the sponges are 



THE MOST PRIMITIVE METAZOA 



71 



therefore more primitive than the coelenterates. Our conclusion, 
therefore, is that the situation is not at all clear. 



(2) The Mesozoa 

The Mesozoa are a group of parasitic animals of very simple 
structure. They are multicellular and usually take the form of a 
solid mass of cells with one or more internal cells. These internal 
cells are not digestive in function but instead play a part in the 
reproduction of the animal. 

The mesozoans in some ways correspond in structure to a solid 
blastula and it has been suggested by some writers such as van 
Beneden (1876) and Hyman (1940) that the Mesozoa are a 
primitive group, or even the most primitive group, of the Metazoa. 
On the other hand the Mesozoa are all internal parasites. Thus 





(a) Ciliated larva 



(b) Female 



(c) Male 



Fig. 25. Mesozoan structure. Rhopalura. 

(a) Ciliated larva. (From Hyman after Atkins.) 

(b) Adult female. (From Hyman after Caullery.) 

(c) Adult male. (From Hyman after Caullery.) 



72 



THE MOST PRIMITIVE METAZOA 



one group, the Dicyemida, represented by Dicyema, is found in the 
kidney of the Octopus, whilst the other group, the Orthonectidae, 
represented by Rhopalura, is found inside various marine inverte- 
brates. Both these groups have a ciliated larva which may bore 
into a new host and this larva in some ways resembles the 
miracidium larva of the digenetic trematodes (Figs. 25 and 27). 
The Mesozoa also have a complex life cycle, and this together with 
the larval structure has led writers such as Stunkard (1954) and 
Caullery (1951) to think that the Mesozoa are probably digenetic 
trematodes. The recent Traite de Zoologie edited by Grasse also 
places the Mesozoa amongst the platyhelminthes. We thus have 
two views concerning the Mesozoa; one that they are primitive 
animals, the other that they are degenerate parasites. 



-INVERTEBRATE HOST- 



Plosmodium >-Agannete- 



Male 



Female 



•Ciliated larva-*- 



- Zygote 



Fig. 26. Diagram of the life cycle of Rhopalura. (After Caullery.) 



It should be stated at the very beginning of this discussion that 
the Mesozoa have suffered as a group in that various non-related 
animals such as Haplozoon have been thrust into the Mesozoa 
though in fact their affinities are elsewhere (see p. 41). There is 
also some doubt about the closeness of the relationship between the 
Orthonectids and the Dicyemids. The resemblance lies in their 
simple morphology and the fact that both have a ciliated larva, 
but since they are both internal parasites, the simple morphology 
is suspect straight away. One knows that other internal parasites 
such as the males of Bonelia, or the parasitic cirripedes, become 
very simplified. On the other hand the adult trematodes and 
cestodes are not morphologically simple — there being a tremendous 
development of the reproductive systems. 

Dodson (1956) has suggested that though a parasitic life can lead 
to morphological simplifications in the parasite, it does not 



THE MOST PRIMITIVE METAZOA 



73 




(a) Infusoriform 
larva 



(b) Older larva 



Fig. 27. Mesozoan structure. Dicyema. 

(a) From Hyman after Nouvel. 

(b) From Hyman after Lameere. 

(c) From Hyman. 












© 



mz 



4 • 



':.©. 



»& 



(c) Nematogen 



necessarily do so. Thus animals such as the leech are parasitic 
but complex in structure, and, as we have already mentioned, 
many of the platyhelminthes and nematodes are quite complex. 
When one also considers the fact that the mesozoans have a 
complex life cycle, some stages of which are not yet known, it 
would appear premature to place the mesozoans in a key position 
between the Protozoa and the Metazoa. 

It is possible to draw up lists of the simple characters and the 
platyhelminth-like characters of the Mesozoa. These are as 
follows. 

Simple characters of the Mesozoa (non-platyhelminth 
characters) 

(1) They are multicellular animals with no differentiation into 
endoderm, ectoderm or mesoderm. 



74 THE MOST PRIMITIVE METAZOA 

(2) They have a solid blastula. 

(3) There is a simple adult form; there are no proglottides, 
no suckers, no thick cuticle, no nervous system, no flame 
cells, no complex gonadial system. 

(4) They have cilia and a few reproductive cells as their 
specialisations. 

(5) The cilia of the trematode miracidium larva are soon lost ; 
those of the Mesozoa last throughout the life of the animal 
(not in the Orthonectids). 

(6) There is no cell in the miracidium comparable to the 
internal nematogen cells of the adult Dicyemids. 

Resemblances between the Mesozoa and the digenetic 
trematodes 

(1) They are internal parasites. 

(2) They have a complex life cycle. 

(3) Both the trematodes and the Mesozoa show polyembryony. 

(4) The trematode miracidium larva and the Orthonectid 
ciliated larva have the following similarities. 

(a) The larva results from similar unequal cleavage of the 
fertilised ovum. 

(b) The larva is bilaterally symmetrical. 

(c) The larva has a fixed number of cells. 

(d) The larva is ciliated. 

(e) The larva does not feed. 

(/) On arrival in the host, the somatic cells degenerate 

and the generative cells develop. 
(g) The larva is the distributive phase between one host 

and the next. 

(5) The adult male Orthonectid has a reproductive duct. 

Details of the structure and life history of the Mesozoa are given 
in Figs. 25-28. Our knowledge of the life cycle of the Ortho- 
nectids is fairly complete but that of the Dicyemids is not. Thus 
we do not know if they have a second host, and, if so, the 
morphology of the parasite in this host. A suggested life cycle is 
shown in Fig. 28 and this cycle is more complex than that 
described for the Orthonectids (McConnaughey 1951). 




THE MOST PRIMITIVE METAZOA 

OCTOPUS- 



75 



Rhombogen 
1 



occasional 



^. Secondary 
Nematogen 




Stem Nematogen 



???+ 



Infusorigen 



[Infuso r ifo"m larva] 



Fig. 28. Diagram of the life cycle of Dicyema. (After 

A IcConnaughey.) 



Caullery (1951) stated that it was probable that the adult of 
Orthonectids such as Rhopalura would on future examination 
show greater histological differentiation. ' A more careful 
histological analysis than so far made will probably disclose a 
nerve ring. What is lacking is a digestive apparatus, as in the 
Monstrillidae, and here this is almost certainly because the life 
of the adult is here even more ephemeral, and entirely devoted to 
the production and dissemination of larvae." Caullery is clearly 
of the opinion that the Orthonectids are degenerate forms. 

There is no certainty that the Orthonectids and the Dicyemids 
are as closely related as their grouping together in the Mesozoa 
suggests. The resemblances are mainly that both have a ciliated 
larva and the adult structure is multicellular without a gut or 
organ systems. The life cycle of Dicyema is not yet fully known 
and so it is difficult to compare it with Rhopalura. There is a 
plasmodial stage in Rhopalura which has not been described for 
Dicyema. The ciliated larva is not identical in structure in the two 
forms. 

If the Mesozoa are not primitive, they can be considered as 
degenerate digenetic trematodes, possibly the miracidium larva 
of some trematode that has become the end stage of development, 
e.g. the Mesozoa are neotenous miracidia. There are, as we have 
seen, certain resemblances between the miracidium and the ciliated 



76 THE MOST PRIMITIVE METAZOA 

mesozoan larva, but as yet no miracidium has been described which 
is as simple as the mesozoan larva. The mesozoan larva has a very 
short life and so it might not have time to develop the flame cells 
found in the miracidium. Perhaps some experimental studies on 
miracidia and the condition under which they can be maintained 
will allow us to come to a greater understanding of the Mesozoa. 
There is also a great deal to be discovered about the life history and 
habits of this group of animals before we can come to any con- 
clusion about their phylogenetic position. 



(3) The Coelenterata 

Over the early years there was a controversy over the animal 
nature of the Coelenterata and it took some time before their true 
animal nature was recognised (Johnstone 1838). Even so, various 
groups of animals such as the Ectoprocta, Endoprocta and the 
Coelenterata were grouped together mainly on the similarities of 
their external form. 

T. H. Huxley in 1849 presented a memoir on the anatomy and 
affinities of the Medusae to the Royal Society of London. In this 
memoir he described how the Medusae differed from the rest of the 
animal kingdom in that they could be regarded as having only two 
layers whilst the other metazoans had three layers. Huxley sug- 
gested that the layers in the Coelenterata were homologous with 
those of the Vertebrata and that in fact the Coelenterata were 
diploblastic. 

In Huxley's text A Manual of the Anatomy of the Inzertebrated 
Animals published in 1891 he modified his views a little. He 
classified the coelenterates into two main groups, the Hydrozoa 
and the Actinozoa, and he included the Medusae in the Hvdrozoa. 

(1) Hydrozoa: (a) Hydrophora Tubularia. 

(b) Discophora Aurelia. 

(c) Siphonophora Physalia. 

(2) Actinozoa: (a) Coralligena Actinia. 

(b) Ctenophora Pleurobrachia. 

He compared the coelenterate body to a sac. " The walls of the 
sac are composed of two cellular membranes, the outer of which is 
termed the ectoderm, and the inner the endoderm, the former 



THE MOST PRIMITIVE METAZOA 77 

having the morphological value of the epidermis of the higher 
animals, and the latter that of the epithelium of the alimentary 
canal. Between these two layers, a third layer — the mesoderm — 
which represents the structures which lie between the epidermis 
and the epithelium in more complex animals, may be developed, 
and sometimes attains great thickness, but it is a secondary and, 
in the lower Hydrozoa, inconspicuous production. Notwithstand- 
ing the extreme variety of form exhibited by the Hydrozoa and the 
multiplicity and complexity of the organs which some of them 
possess, they never lose the traces of this primitive simplicity of 
organisation and it is but rarely that it is even disguised to any 
considerable extent. ... In the fundamental composition of the 
body of an ectoderm and an endoderm, with a more or less largely 
developed mesoderm, and the abundance of thread cells, the 

Actinozoa agree with the Hydrozoa There is a certain similarity 

between the adult state of the lower animals and the embryonic 
conditions of the higher organisations. For it is well known that, 
in a very early state, even of the highest animals, it is a more or less 
complete sac, whose thin wall is divisible into two membranes, 
an inner and an outer. . . . There is a very real and genuine 
analogy between the adult Hydrozoon and the embryonic vertebrate 
animal, but I need hardly say it by no means justifies the assump- 
tion that the Hydrozoa are in any sense ' arrested developments ' 
of higher organisms." 

From the above account by Huxley two points are clear. Firstly 
he thought that the resemblance between the embryonic develop- 
ment of the higher animals and the organisation of the coelenter- 
ates into two main layers of importance as indicating the primitive- 
ness of the coelenterates. Secondly Huxley realised that mesoderm, 
or its precursor, did occur in the coelenterates and thought that 
there was an increase in the thickness and complexity of the 
mesoderm in the higher coelenterates. In effect he assumed that 
simple Hydrozoa such as Hydra and Tubularia were more 
primitive than the members of the Actinozoa. 

There are now two questions that should be considered. The 
first is what are the simple and complex characters of the 
coelenterates? From a study of these it should be possible to 
assess how near the coelenterates are to the basic metazoans. The 
second problem concerns the relationship of the Hydrozoa, 



78 THE MOST PRIMITIVE METAZOA 

Scyphozoa and the Actinozoa, and in effect revolves around which 
of these can be considered as being the most primitive. As we 
have seen, Huxley considered that the Hydrozoa were the more 
primitive, but many zoologists now think that the Actinozoa are 
the more primitive. 

Let us now consider the simple and the complex characters of 
the coelenterates. Some of these, as shown in the following lists, 
are contradictory, but this is due to the wide range of structure 
occurring within the coelenterates. 

Simple coelenterate characteristics 

(1) There are only two well-developed epithelial layers, 
ectoderm and endoderm. 

(2) They have a mesogloea. 

(3) They have musculo-epithelial cells. 

(4) The ectoderm may be ciliated. 

(5) The gut has only one opening. 

(6) There is a hydraulic skeleton. 

(7) They are free living forms and not parasitic. 

(8) Digestion is both intra- and extra-cellular. 

(9) There is no respiratory or excretory system. 

(10) They are polymorphic. 

(11) They have a high regenerative capacity. 

(12) They have a planula larva. 

(13) The nerve net shows little concentration. 

(14) They show radial symmetry. 

Complex coelenterate characteristics 

(1) They may develop cells in the mesogloea to form mesen- 
chyme and mesoderm. 

(2) The body layers may become quite complex, e.g. three types 
of cells in the ectoderm: (a) sensory and mucus cells; (b) 
interstitial cells; (c) muscle cells. 

(3) They may have separate muscle cells (Trachylina and 
Scyphozoa) which may be striated. The musculature can 
be complex, e.g. circular, longitudinal and oblique muscle 
bands. 

(4) They develop a skeletal system. This may be an exoskeleton 
in Obelia or Heliopora or an endoskeleton as in Cor allium. 



THE MOST PRIMITIVE METAZOA 79 

(5) They have a gut. (The Mesozoa and Acoela have no gut.) 

(6) The gut may have subdivisions (pharynx, mesenteries). 

(7) The gut develops a circulation in Aurelia and Alcyonium. 

(8) They have nematocysts. 

(9) They have specialised sense organs such as eyes and 
statocysts. 

(10) Some coelenterates are bilaterally symmetrical. 

(11) Some coelenterates such as Velella and Porpita show 
division of labour. 

As is well known, there is considerable diversity of structure 
within the coelenterates, and even though various coelenterates 
can be derived from a common plan there is still difficulty in 
deciding which are the most primitive coelenterates as opposed to 
the most simple. Thus though one can arrange a series going 
from, say, Hydra to Physalia, or from a diploblastic radially 
symmetrical form to one that is triploblastic and bilaterally 
symmetrical, there is no historical justification for either such 
series. We must find some collateral evidence to help determine 
which are the most primitive of the coelenterates. 

The most primitive coelenterates 

There is hardly a group of the coelenterates that has not at one 
time or another been claimed to have been the most primitive. 
Perhaps the two most prevalent claims are (1) that the polyp is 
the most primitive form, and (2) that the medusa is the most 
primitive form. 

The view that the polyp is the most primitive form in the 
coelenterates has been supported by Haeckel, de Beer and Hadzi 
as well as various other writers. Whilst Haeckel suggests that 
Hydra is primitive, Hadzi thinks that the anemones are more 
primitive and that evolution within the coelenterates has gone 
from the Anthozoa to the Scyphozoa and Hydrozoa. This view 
is supported by de Beer (1954), who writes, " It follows and is 
generally recognised, that the polypoid person which is the only 
one represented in the Anthozoa, is more primitive than the 
medusoid person found in the Scyphozoa and Hydromedusae, 
which is clearly an adaptation to dispersal on the part of the sessile 
form." 



80 THE MOST PRIMITIVE METAZOA 

It is interesting to compare the above statement with one taken 
from Hyman (1940). " The contrary theory, that the ancestral 
coelenterate was a primitive medusa, therefore seems more 
acceptable. This could readily have developed from the meta- 
gastraea by putting forth tentacles and w T hen armed for food 
capture would not have been limited to a bottom habitat." R. C. 
Moore (1956) is of a similar opinion to Hyman. " Next the 
conclusion that the polypoid and medusoid types of organisation, 
instead of representing a more or less unexplained ' alternation of 
generations ' constitute the products of evolutionary differentia- 
tion in which the polypoid form is a persistent early growth, and 
the medusoid is the normal adult type of coelenterate, leads to the 
interpretation of medusoids as the initial type of coelenterate. This 
is consistent with the paleontological record, which includes 
numerous Lower Cambrian and even Precambrian medusoid 
fossils. Consequently the simplicity of the hydroid forms is not 
accepted as a basis for placing them in first position among 
various types of coelenterates. Precedence is assigned to early 
medusoids." 

Though the medusa has been suggested as the basic form in the 
Coelenterata this has not been followed up by claiming that the 
Scyphozoa are the most primitive class of the Coelenterata. The 
life cycle of the Scyphozoa with their dominant medusa and their 
temporary polyp (hydratuba) might fit in with the primitive system. 
The Stauromedusae such as Haliclystus and Lucernaria indicate 
the way in which an adult polyp, even a highly specialised polyp, 
could have arisen. The nematocysts in the Scyphozoa are more 
limited in range of form than those in the Hydrozoa. It might 
be objected that the medusae of the Scyphozoa are very much 
more complex than those of the Hydrozoans, but the complexity 
of the present-day forms does not mean that the original forms were 
of the same complexity. The present-day forms and even the 
Cambrian fossils have a tremendous history of development 
behind them. The choice of a primitive class in the coelenterates 
will clearly depend upon the light that such a choice throws on our 
understanding of coelenterate morphology. 

The most popular choice of primitive class in the coelenterates 
seems to lie between the Hydrozoa and the Anthozoa. Opinion is 
divided as to which of the Hydrozoa are the most primitive. Thus 



THE MOST PRIMITIVE METAZOA 81 

a selection of authors and their choice of primitive form is shown 
below. 

Haeckel Hydrida 

Moser Siphonophora 

Hyman Trachylina 

It is difficult to choose between the above groups; thus though 
Hydra is more simple in its adult morphology, it is suggested 
from a study of the range of form that the medusoid condition is 
more primitive in the Hydrozoa and that development from this 
led to the solitary polyp. 

On the other hand there is a growing body of opinion that the 
Anthozoa are more primitive than the Hydrozoa. This view is 
supported by Hadzi (1944), Ulrich (1950), Remane (1955), 
Jagersten (1955) and Marcus (1958), who suggest the develop- 
mental sequence went Anthozoa-Scyphozoa-Hydrozoa. No 
closely reasoned account has yet been presented by the above 
authors to show exactly how the morphology of the primitive 
anthozoan would lead one to suppose that they are more primitive 
than the hydrozoans, but the gist of the evidence is apparently as 
follows. 

(1) If the Hydrozoa were the most primitive forms which later 
gave rise to the Anthozoa this would not explain the marked 
bilateral symmetry found in the Anthozoa. Bilateral symmetry is 
usually associated with a mobile habit and one would not expect to 
find it in a sessile form that had a long sessile history behind it. 
This bilateral symmetry is found in the Ordovician Tetracorallia 
and even in the arrangement of nagella in the zooxanthella larva. 
From the symmetry as shown in the arrangement of the mesenter- 
ies, the retractor muscles, septal filaments, siphonoglyphs and 
sulcus, one would suppose that the Anthozoa arose from a free- 
living mobile ancestor. 

(2) A second reason for choosing the Anthozoa as the most 
primitive form lies in the range and structure of the nematocysts. 
The Hydrozoa have over a dozen different types of nematocysts 
whilst the Anthozoa have only about half a dozen different types. 
Furthermore the cnidoblast that carries the nematocysts is more 
simple in the Anthozoa; it lacks the cnidocil and instead has a 
primitive ciliary cone (Pantin 1942). 



82 THE MOST PRIMITIVE METAZOA 

(3) The Anthozoa, Scyphozoa and many of the higher animals 
form their endoderm by invagination. This method is rarely 
found in the Hydrozoa, where ingression is more usual, and this 
latter situation has been regarded as being a specialised condition. 

If we accept these reasons for choosing the Anthozoa as the 
primitive class of the coelenterates, what would the primitive 
form look like? It might have been something between an 
Antipatharian and a Protanthean. In the Antipatharia there are 
only six, ten or twelve septa. The longitudinal musculature on the 
septa is scanty or absent and the flagellated tracts are very simple. 
The mesogloea is scanty and the siphonoglyph only weakly 
developed. In Protanthea there are eight macrosepta and four 
microsepta. There is a complete cylinder of longitudinal epidermal 
muscles in the column and pharynx (these are much reduced in 
other Anthozoa). The nerve net and ganglion cells are well 
developed over the surface of the body — the ectodermal nerve net 
being reduced in other anemones. The sphincter and basilar 
muscles are absent. The retractor muscles are weakly developed 
and there are neither septal filaments nor a siphonoglyph. 

Although forms such as Antipathes or Protanthea may be simple 
Anthozoa, they are still very complex when compared to a 
protozoan. We still know very little about the primitive anthozoans 
but it requires a lot of imagination to bridge the gap between the 
Antipatharia and the Protozoa. 

Are the Coelenterata the most primitive of the lower 
Metazoa? We have to choose between the Mesozoa, Porifera, 
Coelenterata, Ctenophora and the Turbellaria to find the most 
primitive metazoan. It seems likely that the simplicity of the 
Mesozoa can be discounted as due to their entirely parasitic 
nature. Similarly the sponges can be discounted since their level 
of organisation is quite different in nature from that present in the 
other metazoan. It would be best to place the sponges on a side 
line to the main line of origin of the Metazoa ; the time of origin 
of the side line is not clear. 

There is no doubt that the simplest of the Hydrozoa are more 
simple than either the Ctenophora or the Turbellaria. But as we 
have already mentioned, we do not know that simple forms such as 
Hydra are the most primitive of the Coelenterata. One of the 
major clues that has been used to place the coelenterates has 



THE MOST PRIMITIVE METAZOA 83 

been that of embryological development. Haeckel suggested that 
the adult coelenterates such as Hydra were at a stage comparable 
to the gastrula seen in Ampkioxus. Even in Haeckel' s time it was 
pointed out that the embryology of the coelenterates did not 
follow that of the higher animal. The blastula of the Hydrozoa 
is most often a solid larva, the interior of which is filled with 
cells. Hadzi and de Beer take this solid larva to indicate that the 
primitive larva had a solid gut and they think that the coelenterates 
cannot be primitive since they have a hollow gut. It may be 
correct that the most common type of planula has a solid 
interior. But this in no way indicates that the adult also had a 
solid gut. 

The Acoela are not the only animals to have a solid gut in the 
adult condition. Within recent years the Pogonophora, a group 
related to the Pterobranchiate Protochordates, have been 
described by Ivanov (1954-7). They have a solid gut filled with 
endoderm cells. The Pogonophora are coelomate animals and it 
is not clear whether the solid gut is here a primitive condition or 
one that is due to the small size of the animal. At any rate it leads 
one to wonder about the precise conditions that lead to the 
retention of the solid gut if it is a primitive condition, or the 
development of a solid gut if it is an advanced condition. Jagersten 
(1955) thinks it highly unlikely that the primitive metazoans had a 
solid gut since this would mean that the hollow gut arose at least 
twice, once in the Coelenterata and again in the Turbellaria. It is 
perhaps worth noting that certain coelenterates such as Clytia, 
when they feed, fill the gastro-vascular cavity with endodermal 
processes so that the gut takes on a solid mesh-like appearance. 
Thus Hadzi's view that the Coelenterata cannot be the most 
primitive of the Metazoa since they have not a solid gut is open to 
two objections: firstly we do not know that the solid gut is a 
primitive condition and secondly some coelenterates can at times 
show a condition resembling a solid gut. 

In conclusion, then, it is apparent that we do not know whether 
the coelenterates are more or less primitive than other lower 
metazoans such as the Turbellaria. We do not know if the hollow 
gut is a primitive condition. We do not know if the Hydrozoa are 
more primitive than the Anthozoa. W T e do not know which is the 
more primitive form, the medusa or the polyp, and as we shall 

7— IOE 



84 THE MOST PRIMITIVE METAZOA 

now see, we do not know the relationship between the Coelenterata 
and the Ctenophora. 

(4) The Ctenophora 

There are three questions that should be discussed concerning 
the Ctenophora. (1) What is their ancestry? (2) Are they 
coelenterates? (3) Are they ancestral Turbellaria? These are all 
difficult questions to answer and involve a careful consideration of 
the structure of the ctenophores. 

The ancestry of the Ctenophora 

Nothing definite is known about the ancestry of the ctenophores. 
It is generally suggested that they arose from a basic stock that 
gave rise to the coelenterates; thus there are certain resemblances 
and certain differences between the ctenophores and the coelenter- 
ates, as can be seen from the lists below. 



Coe 

(1 



enterate characteristics of the Ctenophora 

There are two primary layers ; the ectoderm and endoderm 
are well developed and there is no definite mesoderm — just a 
mesenchyme. 

The main body cavity is the gastro-vascular cavity. 
There is only one opening to the gut — the mouth. There is 
no true anus. 

They have a stomodeum at the entrance to the gut as in the 
anemones and some medusae. 

The gut is divided like that of the Scyphozoa but the eight 
divisions are more like the symmetry of the Alcyonaria. 
They are radially symmetrical. 

They have mesenchyme muscles like some of the coelenter- 
ates, e.g. Trachylina, Scyphozoa. 
The gonads are derived from interstitial cells. 
The outer surface has cilia ; as comb plates in Pleurobrachia, 
as a ciliated surface in Coeloplana. 
The tentacles are like those of some Scyphozoa. 
The lasso cells may take the place of nematocysts, but 
Euchlora has true nematocysts. 
There are no nephridia. 
They have a subepidermal nerve net. 



THE MOST PRIMITIVE METAZOA 85 

(14) Gastrodes has a planula larva. 

(15) Certain coelenterates such as Hydroctena resemble 
ctenophores. 

Differences between Coelenterata and Ctenophora 

(1) Some ctenophores have openings to the gut other than the 
mouth. (Similar openings are found in some medusae such 
as Aequorea.) 

(2) The lasso cells are morphologically quite distinct from 
nematocysts. The nematocysts of Enchlora have been 
stated to have been derived from its food. 

(3) There are no musculo-epithelial cells. 

(4) Coeloplana and Ctenoplana have genital ducts. 

(5) The cilia are arranged in specific rows, or comb plates 
(except for Coeloplana). 

(6) Their symmetry is more biradial than radial. 

(7) The embryology of the ctenophores is determinate and the 
cleavage differs markedly from that of the coelenterates. 

Can the Ctenophora be placed in the Coelenterata? It can 

be seen that there are many resemblances between the coelenter- 
ates and the ctenophores. The two greatest differences seem to lie 
in the possession of nematocysts by the coelenterates and the 
embryology of the two groups. The ctenophores do possess lasso 
cells which differ in their morphology from the nematocysts found 
in the coelenterates. However, one ctenophore, Euchlora rubra, 
has been found to have nematocysts. When these were investi- 
gated by Komai (1942) he found that the nematocysts occurred 
in the tentacles and inside cells which he thought were endodermal 
cells. In 1951 Komai suggested that the nematocysts might have 
been taken from the food of Euchlora since the nematocysts did not 
lie on the surface of the tentacle but were sunk into the ectoderm. 
Hadzi (1951) thought that the nematocysts in Euchlora were 
derived from its food, there being a close resemblance between its 
nematocysts and those of the narcomedusan Cunina. 

Picard (1955) reinvestigated this problem and found that the 
nematocysts did in fact lie on the surface of the ectoderm, correctly 
orientated for discharge. The nematocysts were only found in 
association with ectodermal cells, never endodermal cells. Picard 



86 



THE MOST PRIMITIVE METAZOA 




Fig. 29. Ctenophora. A typical ctenophoran such as Pleurobrachia 
shown here is a round transparent animal with eight ciliated comb 

rows. (From Hyman.) 



suggested that the nematocysts were formed mainly during the 
larval phase of the ctenophore. The nematocysts differ in 
structure from those of the narcomedusae, which would indicate 
that they cannot be derived from Cunina. Furthermore all the 
specimens had nematocysts. 

All these points lead one to conclude that Euchlora has its own 
true nematocysts. This would then indicate that the ctenophores, 
or at least this ctenophoran, belong to the Cnidaria ! 

The apparent wide embryological differences between the 
coelenterates and the ctenophores may be diminished when we 
know more about the range of embryological development of the 



THE MOST PRIMITIVE METAZOA 87 

Scyphozoa. For the ctenophores in many ways resemble the 
scyphozoans; thus, both have a poor regenerative ability in the 
adult, they both develop thick mesenchyme and they both 
develop muscles. Considerable interest was aroused by the dis- 
covery of the medusa Hydroctena. Haeckel suggested that it was 
an ancestral form to the Ctenophora and it certainly shows a 
superficial resemblance to a ctenophore as can be seen from Fig. 31. 
The resemblances are due to the ovoid shape, the two tentacles 
(which Haeckel thought could be retracted into pockets at their 
base) and the gut being divided into four pockets. On the other 
hand there are many differences. Hydroctena has a circular canal 
around the perimeter of its body, the tentacles are oral and non- 
retractile (those of the ctenophores are aboral and retractile). 
There is no statocyst, it has nematocysts and not lasso cells, and 
the gonads develop on the wall of the manubrium instead of the 
radial canals. What is of interest here is the manner in which the 
ctenophoran form can be imitated by a medusoid form. 

Another interesting coelenterate that shows certain ctenophore- 
turbellarian affinities is Tetraplatia. It was classified as a nar- 
comedusan hydrozoan by Carlgren (1926) placed in a separate 
order of the Hydrozoa, the Pteromedusae by Hand (1955), but 
identified as a coronate scyphozoan by Krumbach (1927). Its 
external form is elongate and very much like that of a Muller's 
larva. There are eight lappets around the body, this giving some 
resemblance to a ctenophore; on the other hand the mouth is 
terminal (Fig. 31). Tetraplatia' s affinities are further discussed by 
Ralph (1959). 

It is unfortunate that the planula larva is not more common in 
the ctenophores. The only known case is in Gastrodes, which is 
parasitic on Salpa. Otherwise the ctenophores have a typical 
cydippe larva. There are some clear affinities between the 
coelenterates and the ctenophores but just how closely the two are 
related is hard to say. If further investigations show the presence 
of nematocysts to be more widespread than just in Euchlora and 
if they are of a similar pattern to the coelenterate nematocysts and 
not like those of the protozoan nematocysts, it will indicate that 
the ctenophores can be included in the Cnidaria. The relationship 
between the Hydrozoa, Scyphozoa and Anthozoa seems to be a 
closer one than that of these three to the Ctenophora. 



88 



THE MOST PRIMITIVE METAZOA 




(A). Ctenoplana (dorsal view) 

(After Komai.) 




(B). Ctenoplana (side view) 

(After DawydofT.) 




(C). Coeloplana (dorsal view) 

(After Komai.) 

Fig. 30. Aberrant ctenophores. These ctenophores take up a 
crawling habit and their shape differs from that of Pleurobrachia. 



THE MOST PRIMITIVE METAZOA 



89 





Tetraplatia Hydroctena 

Fig. 31. Aberrant coelenterates. 

Tetraplatia shows superficial resemblance to the Miiller's larva of 
the polyclad platyhelminthes. (After Krumbach.) 

Hydroctena. This medusoid form shows certain resemblances to 

a ctenophore. (After Dawydoff.) 

Relationship of the Ctenophora to the Turbellaria 

Although typical ctenophorans such as Plenrobrachia or 
Hormiphora are round pelagic animals there are some creeping 
forms. It was these creeping forms and in particular Coeloplana 
(Fig. 30) that led Lang (1884) to suggest that the ctenophores gave 
rise to the polyclad Turbellaria. It is not hard to bridge the 
gap between the pelagic forms such as Pleurobrachia and creeping 
forms like Coeloplana. Thus Lampetia is a semi-globular form that 
sometimes crawls on its everted pharynx. Ctenoplana in its 
swimming form is clearly a ctenophore (Fig. 30B) but in its 
crawling form it spreads itself out on its oral lobe and becomes a 
flat animal. Finally Coeloplana is a flattened form like a turbellarian 
(Komai 1922) and it looks very much like a link between the 
ctenophores and the polyclad Turbellaria. In transverse section 
Coeloplana has a complex structure (Fig. 33) and it is not sur- 
prising that Lang thought that it was the forerunner of the 
turbellarians. In particular he associated it with the polyclads 
because of the many branches of the gut. The polyclad resem- 
blances can be seen from the following list. 



90 THE MOST PRIMITIVE METAZOA 

Resemblances between Coeloplana and the Polycladida 

(1) Both have a flat, compressed body. 

(2) They move by creeping on the sole of the " foot." 

(3) The body surface is ciliated. 

(4) There is a well- developed basement membrane. 

(5) The dermal musculature is well developed; the dorso- 
ventral muscles may be branched. 

(6) The gastric canals have many branches; there is no anus 
but some pores end externally. 

(7) There is a stomodeal invagination on the ventral surface. 

(8) Both show determinate cleavage. 

(9) The large micromeres give rise to small micromeres. 

(10) The micromeres form the ectoderm. 

(11) There is no hollow blastula stage. 

(12) The development is mosaic. 

(13) The mesoderm arises from the macromeres. 

(14) The embryo gastrulates by epiboly. 

(15) The Muller larva present in some polyclads has eight 
ciliated lappets. 

(16) There is a small apical nervous tuft. 

(17) There is a statolith. 

(18) Both groups have paired tentacles. 

(19) There are gonadial canals. 

The above list is impressive in length and indicates a consider- 
able similarity between the two groups. Lang suggested that the 
centrally positioned nerve centre in the ctenophores moved 
anteriorly to take up the typical polyclad position, i.e. a change 
from biradial symmetry to bilateral symmetry (Fig. 32). The list 
may also demonstrate another point. Often in discussing such a 
problem, a list of characters is drawn up of the points for and the 
points against a given viewpoint. The difficulty comes when one 
has to decide the relative importance of each character. It is 
impossible to come to a decision just by seeing whether there are, 
say, more similarities than differences. Instead each point must be 
weighted according to its importance and this is difficult since the 
importance often reflects the opinion of the observer. 

In spite of the similarities between Coeloplana and the Poly- 
cladida, the general opinion these days is that the similarities are 



THE MOST PRIMITIVE METAZOA 



91 




(A). Ctenophoran condition 




(B). Hypothetical condition 




(C). Polyclad condition 

Fig. 32. Lang's concept of the manner in which the ctenophores 

could have given rise to the polyclads. The anus moved ventrally 

and backwards and the body form became elongated. 



92 



THE MOST PRIMITIVE METAZOA 



due to convergence and that Coeloplana and Ctenoplana are in fact 
specialised aberrant and advanced forms. In particular there are 
considerable differences between the embryology of the cteno- 
phores and the polyclads which make it improbable that the two 
groups are related. These differences are not due to the presence 
or absence of yolk but instead reflect a more fundamental differ- 
ence. The ctenophore egg cleaves into four and at the next 
cleavage it divides to form a small group of cells, the micromeres, 
and a large central group, the macromeres. These form a flat 
plate of cells. Cleavage continues till there are eight macromeres 
and many micromeres. In the polyclads, on the other hand, after 
the four-cell stage the embryo shows a definite spiral cleavage 
pattern. The cells can all be classified in terms of the spiral cleavage 
pattern found in the Rhabdocoelida, Tricladida, Annelida and 
Mollusca. This spiral cleavage is not found, nor is there any 
indication of spiral cleavage, in the development of the 
Ctenophora. 

There are other differences between the polyclads and the 
ctenophorans. Thus the polyclads have a well-developed brain, 



mesenchyme 



lumen of 
food canal 



muscle fibre — 

eosinophil_ 
body 




° O o 

1 Q.° J. o o ,o 



• o o ° • -P • » ° °„ 

° O • n r, °"o ° n° °»° 




gland cell-- -£- 

ciliated 
epidermis 

Fig. 33. Diagrammatic transverse section through the body of a 
ctenophore, Coeloplana. (After Komai.) 



THE MOST PRIMITIVE METAZOA 



93 



often the most highly developed brain of all the turbellarians ; 
they also have well-developed and numerous eyes, flame cells, 
a complex reproductive system with a muscular penis, uterus, 
seminal vesicle and prostate organ, and often show hypodermic 
impregnation. The Ctenophora have nothing to compare with 
this. 

It is interesting to mention here that Hadzi (1944) and de Beer 
(1954) think that the Ctenophora arose from the Polycladida, a 
viewed discussed in more detail on page 94. Hadzi places the 
Ctenophora in the platyhelminthes though he agrees that 
Coeloplana and Ctenoplana are aberrant forms. 

What conclusion can we come to regarding the position of the 
Ctenophora? With regard to their level of organisation they are 
in most respects at a similar level to that seen in the Anthozoa- 
Scyphozoa line of the Coelenterata. It is not possible to place 
them any closer than this until more research has been carried out 
on the embryology and development of the ctenophores and until 
the range of form of the coelenterates is better known. It is not 



' 1 j.lll L ! J J-l? - 1 .'.'. J J.U^ jJJJLL'-lLU-'-'-lJjj J-t JJji J JiJJJ JJJJ.J. 1 . 1 LLJAl i ! U-'.'.'IV. ciliated epithelium 




glandular cell 
mesenchyme 



food vacuole 



— muscle fibre 



rr~ — "~ 



pharynx 



Fig. 34. Diagrammatic transverse section through the body of an 

Acoelan, Convoluta. There is a certain resemblance to the grade 

of organisation of the ctenophorans. (After von Graff.) 



94 THE MOST PRIMITIVE METAZOA 

even certain at this stage that the ctenophores and coelenterates 
had a common origin. It is possible that the nematocysts of the 
ctenophores could have arisen independently of those in the 
coelenterates ; after all there are some well-developed nematocysts 
in the Protozoa. On the other hand there is almost nothing to 
favour the view that the ctenophores are platyhelminthes. This is 
particularly so because the turbellarians have a well- developed 
reproductive system with accessory muscular sacs, whilst the most 
that any ctenophore has is a small reproductive duct. There is 
thus no clear indication that the ctenophores either gave rise to 
or were derived from the Turbellaria. 



(5) The Platyhelminthes 

Though the platyhelminthes are usually considered as having 
evolved after the Coelenterata, Hadzi (1944, 1953) has suggested 
that this is not the case and that in fact the coelenterates evolved 
after and from the platyhelminthes. The classification that Hadzi 
gives of the lower Metazoa is as follows. 



Phylum 


Subphylum 


Class 


Spongiaria 


Spongiaea 




Ameria 


(1) Platyhelminthes 


Planuloidea 

Turbellaria 

Ctenophora 

Trematoda 

Cestoda 




(2) Cnidaria 


Anthozoa 

Scyphozoa 

Hydrozoa 



This classification differs from the usual one in several respects. 
First of all the platyhelminthes are considered to be the most 
primitive of all the Ameria, more primitive than the Cnidaria. 
Secondly the Ctenophora are placed in the platyhelminthes. 
Thirdly the Anthozoa are considered to be the most primitive of 
the Cnidaria. The evolutionary sequence devised by Hadzi is as 
follows. 



THE MOST PRIMITIVE METAZOA 



95 



Hydrozoa 

t 

Anthozoa 

t \ 

Ciliata — > Acoela -> Rhabdocoelida Scyphozoa 

\ . 

Polycladida — > Ctenophora 

Hadzi thinks that the Metazoa arose by the formation of cell 
walls in a syncytial ciliate. This would then lead to a multicellular 
animal with a complete organisation, i.e. an antero-posterior axis 
and without the difficulties of reorganisation that a multicellular 



brain 



glandular 
secretion 



.statocyst 



0" 



p&, 






:'Lfood vacuole 



mesenchyme 






Ff^:':7. v.-.SM: : - ift- '.•':'•• ' : , --mouth 
• ■ ■■ •'• & ••':..•. • ••• :•",.. • ■A.I 



food-5 
vacuole 



oenis 




ovary 

bursa 
seminalis 



___ Reproductive 
opening 



cilia 



Fig. 35. Diagrammatic longitudinal section through an Acoelan 
to show the order of complexity of its structure. Note the 
development of a complex reproductive system. (From Bronn.) 



96 THE MOST PRIMITIVE METAZOA 

colonial animal might have had. The syncytial level of organisa- 
tion would correspond to the solid blastula, the stereoblastula, 
that is sometimes found in the embryology of the Metazoa. The 
simplest Metazoa did not have a hollow gut and corresponded in 
structure to the present-day Acoela (Fig. 35). 

The Acoela gave rise to the other platyhelminthes, amongst 
which were the Rhabdocoelida with their straight gut, and the 
Polycladida with their branched gut. The Rhabdocoelida gave 
rise to the Anthozoa which in their turn gave rise to the Hydrozoa 
and the Scyphozoa. The Polycladida have a swimming larval 
form, the Miiller's larva, and this became neotenous and gave 
rise to the Ctenophora. Neotenous Miiller's larvae have been 
described; thus Heath (1928) described Grafizoon lobata which 
resembled a sexually mature Miiller's larva. 

The arguments that Hadzi puts forward in favour of his views 
are as follows and he is supported by de Beer (1954, 1958). 

The Coelenterata are not primitive 

(1) The radial symmetry shown by the coelenterates is second- 
arily acquired by them. The development of bilateral symmetry is 
shown first of all in the external parts of the higher animals and is 
then impressed on the internal organs. In the coelenterates such 
as the Anthozoa, the bilateral symmetry is only found internally 
as in the mesenteries. Therefore it must have lost its external 
bilateral symmetry and be in the process of acquiring a radial 
symmetry concomitant with a sessile habit. 

(2) The polyp is more primitive than the medusoid form. 
Since the polyp is found in the Anthozoa whilst the Hydrozoa have 
both polyp and medusa it follows that the Anthozoa are the most 
primitive of the Coelenterata. 

(3) The Coelenterata are not diploblastic. They have a well- 
developed middle layer, the mesogloea, which often contains 
cells. Furthermore the cell lavers in the Anthozoa and the 

j 

Hydrozoa are not strictly comparable since in the Hydrozoa the 
germ cells are formed from the ectoderm whilst in the Anthozoa 
and the Scyphozoa they are formed from the endoderm. The 
germ layers in the coelenterates are not comparable to the germ 
layers of the higher animal and are not even homologous within 
the coelenterates. 



THE MOST PRIMITIVE METAZOA 97 

(4) Haeckel suggested that the Coelenterata represented the 
hollow gastrula stage found in the embryology of the Echinoderms, 
Sagitta and Amphioxus. This would indicate that the coelenterates 
are a primitive group. But we now know that the blastula is more 
often a solid form and that the endoderm is not always formed by 
the invagination and development of a hollow gut. In fact the 
hollow gut is an advanced condition when compared with that 
present in the Acoela. 

Objections can be raised to all of Hadzi's points. 

(1) There is no evidence that the radial symmetry is secondarily 
acquired by the coelenterates. The detection of an external 
symmetry depends upon having some external organs; the 
coelenterates do not have any such organs along the length of the 
polyp and thus they cannot display this external bilateral sym- 
metry. It cannot therefore be established that they had an 
external bilateral symmetry at some stage which was later lost. 
Furthermore there is no evidence that bilateral symmetry is 
acquired first of all by the external organs and later by the internal 
organs. 

(2) It is not generally accepted that the polyp is the most 
primitive form in the coelenterates. In fact there is quite a 
body of opinion that holds the medusa to be the primitive form 
(see p. 80). 

(3) Though it is correct that not all the coelenterates are 
diploblastic, the mesogloea of many of the Hydrozoa shows little 
or no development. They at least can be considered as having an 
effective diploblastic condition. As for homologising the cell 
layers, it is not strictly correct to assert that the gonads arise 
from the ectoderm in the Hydrozoa. It is more correct to state 
that they arise from interstitial cells. In this way, therefore, one 
can homologise the germ layers within the coelenterates. 

(4) Finally the fact that the blastula may often be solid in no 
way indicates that the adult must have had a solid gut in 
the most primitive metazoans. The larval form merely indicates 
what the primitive larval condition was like, not the adult 
condition. 

Having asserted the non-primitive nature of the coelenterates 
Hadzi presents the following evidence that the Acoela are more 
primitive than the coelenterates. 



98 THE MOST PRIMITIVE METAZOA 

The Acoela are primitive 

(1) The Acoela organisation corresponds to that of a ciliate 
that has developed cell walls. 

(2) The Acoela have no gut. 

(3) The Acoela are hermaphrodite and have internal fertilisa- 
tion rather like the syngamy of the ciliates. 

(4) The Acoela often have a syncytial gonad, epidermis, repro- 
ductive system and digestive system. This is a relic of the 
original ciliate syncytium. 

(5) The digestive system of the Acoela can be derived from the 
protozoan food vacuoles (Figs. 5 and 35). 

(6) The nephridial system can be derived from the protozoan 
contractile vacuoles. The Acoela do not have flame cells. 

(7) The Acoela usually have a ciliated ectoderm. 

(8) The Acoela often have musculo-epithelial cells. 

(9) The Acoela have no basement membrane. 

(10) The Acoela have a central mouth like the ciliates. 

(11) The Acoela have a simple pharynx derived from a 
stomodeum. 

(12) There are no distinct gonads. 

(13) The rhabdites are derived from the trichocysts. 

The Acoela and the Polycladida differ in their morphology and 
development from the rest of the Turbellaria. Hadzi suggests that 
the Acoela gave rise to the Rhabdocoelida and the Polycladida. 
The Rhabdocoelida then gave rise to the Anthozoa by the loss of 
their protonephridia, the reduction of their nervous system, the 
simplification of their digestive system, the loss of accessory 
reproductive organs and reduction of the mesoderm. The slime 
glands of the epidermis of the rhabdocoels gave rise to the 
nematocysts. 

The derivation of the Anthozoa from the Rhabdocoelida in this 
way would be very surprising with no other parallel in the animal 
kingdom, i.e. reduction and simplification giving rise to a whole 
phylum of widely diverse and successful animals. This does not 
mean that such a reduction is impossible; it just seems highly 
improbable. It also seems unlikely that the polyclads gave rise to 
the ctenophores. 



THE MOST PRIMITIVE METAZOA 99 

There is also little assurance that the Acoela are the most 
primitive of the Turbellaria. They are a comparatively unstudied 
group of animals; we know next to nothing about their physiology, 
little experimental embryology has been performed on them and 
we do not know their range of morphological forms. Though von 
Graff (1904) was of the opinion that their structure was primitive, 
a great deal more research will have to be carried out before such a 
position can be justified and even more research will be necessary 
before they can seriously be derived from the ciliates. For 
further details concerning the phylogeny of the platyhelminthes 
and their relationship to the Ctenophora one should consult 
Bresslau (1933). 

There is a school of thought represented by Marcus (1958) which 
suggests that the platyhelminthes are an advanced group of 
animals that were once coelomate and more complex morphologic- 
ally than, say, the Nermertini, Phoronidea or Brachiopoda. The 
platyhelminthes according to this view are secondarily simplified. 
They have lost their coelom, anus and circulatory system. They 
have reduced their nervous system, and altered their reproductive 
system to make up for the loss of the coelom. It is suggested that 
one well-known example of a coelom being lost when an animal 
takes up parasitic habit is that of the Hirudinea, and that the 
platyhelminthes have gone even farther along this course. 

We thus have two conflicting views concerning the status of the 
platyhelminthes. Hadzi suggests that they are the most primitive 
of all the Metazoa, being more primitive than the coelenterates 
and derived from the ciliates. Marcus suggests that the platyhel- 
minthes are an advanced group of animals whose simplicity of 
structure is due to their parasitic habit and that they arose some 
time after the Nemertea. 

What can one conclude about the most primitive of the Metazoa? 
There are, as we have seen, five contestants, Porifera, Mesozoa, 
Coelenterata, Ctenophora and the Platyhelminthia, for this title. 
These groups are almost completely isolated from each other 
though a few tenuous connexions can be made. It is quite clear 
that the available evidence is insufficient to allow us to come to 
any satisfactory conclusion regarding their interrelationships. At 
the same time it is also clear that a great deal of work remains to 
be done on all of these groups. We are still very ignorant about the 

8— IOE 



100 THE MOST PRIMITIVE METAZOA 

comparative physiology and biochemistry of the lower Metazoa 
and very little experimental work has been done on their 
embryology. It is possible that these lines of research will help 
in the elucidation of the relationships between the lower Metazoa. 
It is also possible that the new information will indicate more 
clearly that the Metazoa are polyphyletic. 



CHAPTER 7 



THE INVERTEBRATE PHYLA 



Within the invertebrates there are many distinct phyla. So far 
we have considered some of the possible relationships between the 
so-called " lower phyla," namely the Protozoa, Porifera, Mesozoa, 
Coelenterata, Ctenophora and Platyhelminthia. There are, how- 
ever, many other important phyla such as the Nematoda, Nemertea, 
Rotifera, Annelida, Arthropoda, Mollusca, Brachiopoda, Echino- 
dermata and Protochordata that all deserve some mention for they 
each present special problems of phylogenetic relationship. 

It is not possible to obtain satisfactory palaeontological data 
concerning the relationship of these various phyla because most 
of them are already fully established in the earliest fossil-bearing 
beds, the Cambrian. This means that one has to use other inform- 
ation to determine the relationships between these phyla. In 
fact these relationships are not at all clear and this can best be 
illustrated by examining three attempts that have been made to 
present a coherent monophyletic relationship of the major 
invertebrate phyla. 

Grobben's Classification 

Karl Grobben in 190S proposed a scheme to show the inter- 
relationship of various invertebrate groups. This system has 
formed the basis for most of our current schemes, e.g. Cuenot 
1952. The system divides the major invertebrate phyla into two 
sections, the Protostomia and the Deuterostomia. This distinction 
had been proposed by Goette in 1902 and was based on the fate 
of the blastopore in the developing embryo: whether it becomes 
the anus or the mouth and anus. In the Protostomia the blastopore 
becomes the mouth and anus whilst in the Deuterostomia it 

101 



102 THE INVERTEBRATE PHYLA 

becomes the anus; the mouth develops in another position. 
Grobben then divided the phyla in the following fashion. 

Protostomia Deuterostomia 

Scolecida Chaetognatha 

Molluscoidea Echinodermata 

Mollusca Enteropneusta 

Annelida Tunicata 

Arthropoda Acrania 

Vertebrata 

By Scolecida, Grobben meant the Platyhelminthia, Entoprocta, 
Aschelminthia and Nemertini. In the term Molluscoidea he 
included the Phoronidea, Ectoprocta and Brachiopoda. Sometimes 
the group Molluscoidea was referred to as the Tentaculata. 

Let us first of all consider the validity of these two major groups, 
the Protostomia and the Deuterostomia. In the Protostomia the 
situation is not as clear cut as the classification might suggest. 
Thus in the platyhelminthes, the blastopore closes in Convoluta 
and Planocera and the mouth is a new formation (there is of 
course no anus in the platyhelminthes). In the polyclads the 
original blastopore closes and disappears but the pharynx develops 
near the site of the erstwhile blastopore. In the Tardigrada the 
blastopore does not develop. In the Entoprocta the blastopore 
closes and a new mouth and anus develop. In the Annelida the 
situation varies according to the animal studied. In Nereis and 
Podarke the blastopore forms the mouth and anus in the required 
manner. In Pomatoceros the blastopore forms the mouth but the 
anus is a new formation. In Capitella, Ctenodrilus and Saccocirrus 
the blastopore closes and the mouth and anus are new formations. 
In the oligochaete Dendrobaena the anus is a new formation and it 
is not derived from the blastopore. 

In the Arthropoda the situation is much the same. In Peripatus 
capensis the blastopore after a brief closure opens again to form 
the mouth and anus. In some crustaceans such as Caridina the 
anus develops some distance away from the blastopore region 
whilst the mouth develops as a new formation unrelated to the 
blastopore. In Astacus the proctodeum arises from the region near 
the site of the blastopore. 



THE INVERTEBRATE PHYLA 103 

In the Mollusca the fate of the blastopore also varies. In the 
Gastropoda as a rule the anterior part of the blastopore gives rise 
to the mouth, in Paludina the mouth arises from the posterior 
part of the blastopore. In all gastropods the anus is a new 
formation unrelated to the blastopore. In the Amphineuran 
Ischnochiton, too, the anus is a new formation. The lamellibranches 
such as Teredo or Cyclas have the blastopore closed completely 
and the mouth and anus are entirely new formations. Otherwise 
the blastopore becomes the site of the mouth. Further informa- 
tion can be found in DawydorT (1928) and Manton (1948). 

In the Deuterostomia the fate of the blastopore is similarly 
varied. The echinoderm mouth is a new formation in the larva 
whilst the blastopore becomes the larval anus. The same is true 
for the hemichordates. In the Tunicata and the Cephalochordata 
the blastopore becomes dorsally placed to form the neuropore. In 
the Chaetognatha the blastopore closes and does not become 
either the mouth or the anus. 

It can be seen that the division into Protostomia and Deuteros- 
tomia is not as sharp as might be expected. Thus certain groups 
such as the Tardigrada, Chaetognatha, Tunicata, Cephalochordata 
and so on would be in neither the Protostomia nor the Deuteros- 
tomia. In other groups such as the Annelida or Arthropoda, 
certain genera have the blastopore forming the mouth whilst 
others do not. 

This situation was appreciated quite early. Thus Sedgwick 
stated in 1915, " In Peripatus the mouth and anus are not only 
derived from the elongated blastopore by its constriction into two 
openings but remain throughout life included within the nerve 
ring derived from the neural rudiments of the embryo. If in 
other Arthropoda, in Annelida, and in the Mollusca we find, as 
we do, that the nerve ring referred to is, in the adult, incomplete 
behind the anus, and the mouth and anus, though obviously 
referable to the blastopore, are not actually derived from it, must 
we on this account deny this most obvious relation and maintain 
that the mouth or anus, as the case may be, in these forms is not 
homologous with that of Peripatus} To maintain such a position 
appears to us impossible and we entirely accept the doctrine that 
the mouth and anus of the Annelida, Arthropoda, and Mollusca 
are both perforations of the embryonic neural surface and are 



104 THE INVERTEBRATE PHYLA 

Tunicata Arthropoda 

Acrania / Chaetognatha Echinodermala 
Vertebrata 




Enteropneusta 




Mollusca 



Annelida 



(Chordonia) ^omalo- ( Ambulocralia) 
pterygia) 




Spongiaria 




Molluscoidea 



(Deuterostomia) 



Ctenophora 



(Coelenterata) 
Metazoa 



Rhizopoda 



Sporozoa 




Ciliata 
(Cytoidea) 



Flagellata 
(Cytomorpha) 

Protozoa 
Fig. 36. Grobben's classification of the Invertebrate Phyla. 



specialisations of parts of one original opening which is repre- 
sented in most embryos by the blastopore. 

" When, however, we come to apply this doctrine to the Chordata 
we stand on more debatable ground. Placing the Enteropneusta 
on one side as not obviously conforming to our plan, we find that 
it is a fact of observation that in the Chordata the blastopore 



THE INVERTEBRATE PHYLA 105 

perforates the embryonic rudiment and that in some of them the 
anus is directly derived from it. (Many Pisces, some Amphibia, 
e.g. Newt.) Whereas in others not at all remote from these, the 
blastopore closes entirely and the anus is a new formation (some 
Pisces and Amphibia, e.g. Frog, Amniota). Here also we think 
it may be fairly maintained that notwithstanding the diversity in 
the mode of development of the anus, it is, in all vertebrata at 
least, a derivative of the blastopore." 

It will be seen from Sedgwick's account that whilst he realised 
there was considerable diversity in the fate of the blastopore, he 
thought the generalisation — that the blastopore became the 
mouth in the Protostomia and the anus in the Deuterostomia — a 
fair one. On the other hand Manton (1948) thinks that the 
variability in the mode of development of the mouth and anus is so 
great in the Annelida and the Arthropoda that it no longer forms a 
useful link between these two groups. " It is clear that most of the 
known species of Onychophora fall into line with the Arthropoda 
in the dissociation of the mouth and anus from the blastoporal 
area and contrast with the majority of the Polychaeta." 

There are other characters that can be used to separate the 
Protostomia and the Deuterostomia, or the Annelid and the 
Echinoderm Superphylum as they are sometimes called. 

Annelid Superphylum Echinoderm Superphylum 

Spiral cleavage Radial cleavage 

Blastopore = mouth Blastopore = anus 

Schizocoelic coelom Enterocoelic coelom 

Determinate cleavage Indeterminate cleavage 
Nervous system delaminates Nervous system invaginates 

Ectodermal skeleton Mesodermal skeleton 

Trochosphere larva Pluteus type larva 

To each and every one of these characters many exceptions 
can be found and in particular, certain groups of animals seem to 
lie between the two superphyla. Thus the Brachiopoda have their 
blastopore forming the mouth, their coelom is enterocoelic and 
their cleavage is of the radial type. The situation is most difficult 
for the Nematoda, Ectoprocta and Phorodinea and these are the 
groups that one would most like to place accurately. Even within 



106 THE INVERTEBRATE PHYLA 

the major groups there is some disagreement. Thus Raven in his 
account of morphogenesis in the Mollusca states that it is incorrect 
that the group as a whole shows determinate cleavage. This would 
indicate that the Mollusca-Annelida link is not necessarily as 
close as some authors imagine. On the other hand it is necessary 
to keep some sense of balance and not lose sight of the wood 
because of the trees. 

Grobben's classification is shown in Fig. 36. In some ways it 
resembles the next classification to be discussed, that of Marcus, 
but there are certain differences. Thus the Coelomata arise from 
the line that led to the Ctenophora. The Enteropneusta are more 
allied to the Echinodermata than they are to the Tunicata or 
Vertebrata. 

Marcus's Classification 

This view of the phylogeny of the invertebrates has been 
described by Marcus (1958) and it agrees in many ways with that 
described by Grobben and also with that described by Ulrich 
(1950) and Remane (1954). Marcus considers that the Anthozoa 
are the most primitive of the Coelenterata. All the forms above 
the Coelenterata are called " Bilateria " since they are almost all 
bilaterally symmetrical. They are also called " Coelomata " and 
Marcus considers that all these forms are derived from an 
ancestor that had the " fundamental features of the Archicoelomata, 
viz. three coeloms, mouth, anus, vessels and perhaps tentacles." 
The coelom was developed as a series of pouches from the gut, as 
suggested by Sedgwick (1884), and the Bilateria could have arisen 
from either the Anthozoa or the Ctenophora. 

Since the Bilateria are all coelomate this means that the 
Platyhelminthia, Rotifera, Nematoda and Endoprocta all are 
derived from a form that once had a coelom. During the course 
of evolution the coelom became reduced in these forms till some- 
times all that is left is the cavity of the flame cells. The resemblance 
that has been reported between the planula larva and the Acoela, 
Marcus suggests, is entirely due to the small size of the animals. 

The Bilateria are divided by Marcus into the Protostomia and 
Deuterostomia, as we have already seen in Grobben's classification, 
the only addition being the newly described Pogonophora, which 
are placed in the Deuterostomia. Marcus points out that there 



THE INVERTEBRATE PHYLA 



107 



are certain real resemblances between the animals at the base of 
the Protostomia and the base of the Deuterostomia. Thus the 
Ectoprocta and the Pterobranchiata have a body that is in three 
segments; the Ectoprocta and Brachiopoda are enterocoelic, the 
Ectoprocta have coelomic pores and budding is often similar in 
pattern. 

The first major division within the Protostomia is the Tenta- 
culata, which is comparable to the Molluscoidea of Grobben. 
Of these the Phoronidea are considered as being the most primitive 
whilst the Entoprocta are considered to be derived from attached 
larvae of the Ectoprocta. 

The Nemertea are coelomate, their coelom being restricted to 
the rhynchocoel and the gonad cavities. The nemerteans are more 
primitive than the Turbellaria since they have an anus and their 






Aschelminthes 



f Arthropodo I 

V. v J 



^Annelida A, || usca 



Platyhelminthes 
Nemertini 




Fig. 37. Marcus's classification of the Invertebrate Phyla. 



108 THE INVERTEBRATE PHYLA 

genitalia are more simple. Their embryology indicates a possible 
relationship with the polyclads. 

The Platyhelminthia have most of their organ systems reduced 
and simplified; the animals are not therefore primitive. The 
coelom is reduced to the ciliated ducts of the reproductive organs. 
The Acoela are not the most primitive of the Turbellaria (Fig. 37). 

The Aschelminthia include the Nematoda, Rotatoria, Gastro- 
tricha, Nematomorpha, Kinorhyncha and Priapulida, some of 
which show a spiral pattern of cleavage. It is not clear if the 
Aschelminthia are a closely related group of animals. 

The Mollusca and Annelida are derived from a common 
ancestry. The ventral pharyngeal sac of the archiannelids is 
similar to the radula sac of the molluscs and the teeth of the 
Eunicidae show plates that are similar to the radula teeth. The 
primitive molluscs such as Neopilina may be segmented. 

The Articulata (Arthropoda) arose several times from the annelid 
stock. The Pentastomida, Onychophora and Tardigrada are three 
groups that are quite distinct from one another, though similarities 
between the legs, body cavity and gonads can be used to form a 
link between the Tardigrada and the Onychophora. The Trilobita 
gave rise to the Arachnomorpha. The crustacean resemblances 
of the trilobites are due to homoiology; the independent deriva- 
tion of similar structures in separate lines that are phylogenetically 
related. (Other examples of homoiologous organs are compound 
eyes, trachea and malpighian tubules.) The basic line that gave 
rise to the Crustacea also gave rise to the Antennata from which 
came the Myriapoda and the Insecta. 

The Deuterostomia are a smaller and more compact group than 
the Protostomia. The hemichordates contain the Enteropneusta 
and the Pterobranchiata. The Enchinodermata and the Enter- 
opneusta are linked together by the dipleurula larva. The 
ancestor of the Hemichordata then being postulated as giving rise 
to the Tunicata and the Vertebrata. 

Hadzi Classification 

The relationship that Hadzi (1944, 1957) postulates between 
the various invertebrate groups can be seen from Fig. 38. He 
derives the Metazoa from the Ciliophora. In the ciliates there is 



THE INVERTEBRATE PHYLA 



109 



Mommolia 



Reptilia- 
Amphibia - 



Insecta 




Apterygogenea 

Diplopoda 
Chilopoda 
Protracheata 
Arthroppdq 

Arachnoidea 

Xiphosura 

Pantopoda 

Crustacea 



Kamptozoa 
Nematomorpha 

Nematodes 
Acanthocephala 
Priapuloidea 



Sporozoa 
Rhizopoda 

Flagellata 
Fig. 38. Hadzi's classification of the Invertebrate Phyla. 



often a differentiation of the cytoplasm in a manner that can be 
compared with the ectoderm, mesoderm and endoderm of the 
metazoa. Hadzi thinks that such a ciliate gave rise to a form 
resembling an acoelous turbellarian and that the Turbellaria are 
the most primitive of the Metazoa. The Turbellaria then gave 
rise to the Anthozoa, as has been mentioned on p. 95. 

The coelom of the metazoa is traced back to the mesohyal 



110 THE INVERTEBRATE PHYLA 

(mesoderm) of the ciliates; it is not therefore a new formation. 
Various cavities such as the nephridial cavity, blood cavity, 
secretory cavities, lymph cavities, rhynchocoel and pericardium 
are all regarded as being part of the coleomic system. There are 
primary cavities, those without an epithelial lining, and secondary 
cavities, those with an epithelial lining. The perigastrocoel is a 
space lying alongside the gut and it too has become lined with 
epithelium. Those animals that have such a perigastrocoel and 
which are also unsegmented Hadzi places in his first metazoan 
phylum, the Phylum Ameria. Included in the Ameria are the 
following groups: Platyhelminthia, Coelenterata, Gastrotricha, 
Rotatoria, Kinorhyncha, Mollusca, Priapuloidea, Acanthocephala, 
Nematoda, Nematomorpha, Nemertini and Kamptozoa. 

The second phylum is the Phylum Polymeria. These animals 
are all segmented and the perigastrocoel becomes initially broken 
up to form a series of cavities. In some of the higher Polymeria 
the cavities become reduced, as in the Hirudinea. Included in 
the Polymeria are the following groups : Annelida, Sipunculoidea, 
Echiuroidea, Crustacea, Pantapoda, Xiphosura, Arachnida, Chilo- 
poda, Diplopoda and Insecta. 

The third phylum is the Phylum Oligomeria. These animals 
have at some stage of their development adopted a sessile habit 
and this has led to a reduction in body segmentation. In the 
Oligomeria are placed the Phoronidea, Brachiopoda, Bryozoa, 
Chaetognatha, Echinodermata, Pogonophora, Enteropneusta and 
Pterobranchiata. 

The fourth phylum is the Phylum Chordonia. In this are placed 
the Vertebrata and the Tunicata. 

Hadzi does not consider that the higher invertebrates can be 
satisfactorily classified into Protostomia and Deuterostomia. 
Instead he thinks that the line that gave rise to the higher 
arthropods also gave rise to the echinoderms. 

Marcus's classification and that of Grobben have more in 
common than either has to Hadzi's, the greatest difference being 
in the position of the platyhelminthes ; Grobben thinks they are 
primitive, Marcus thinks they are advanced. The major difficulty 
in their classification lies in the placing of phyla other than the 
Annelida, Mollusca, Echinodermata and Protochordata. All the 
remaining small phyla are difficult to place. It is possible that some 



THE INVERTEBRATE PHYLA 111 

of them had an independent evolution from the Protozoa. Even 
within the major groups such as the Arthropoda difficulties arise ; 
it is becoming more certain that the Arthropoda are not a mono- 
phyletic phylum of animals but instead are a grade of organisation 
and that this grade has been reached independently many times 
from some annelid-like stock (Tiegs and Manton 1958). 

It would appear that the relationship between the various 
invertebrate phyla is a very tenuous one. There are many phyla 
that seem to be isolated from each other, and even those phyla 
that seem reasonably close to one another, on detailed examination 
show differences as important as their similarities. Though it is 
useful to consider that the relationships determined by com- 
parative anatomy and embryology give proof of a monophyletic 
origin of the major phyla, this can only be done by leaving out 
much of the available information. Let us now consider the 
invertebrate relationships determined by comparative biochemistry 
and see if they lead to any more definite conclusions. 



CHAPTER 8 

BIOCHEMICAL STUDIES OF 
PHYLOGENY 



The previous discussions concerning the phylogeny of animals 
has been concerned with evidence based mainly on morphological 
data. Within recent years, however, biochemical studies have 
been used to help determine animal relationships and the results 
so obtained have aroused considerable interest. Only two such 
studies will be considered here: those concerned with the dis- 
tribution of phosphagens and those concerned with the distribution 
of sterols through the animal kingdom. 

Due mainly to the work of the Cambridge biochemists, con- 
siderable interest has been focused on the phosphagens present 
in the invertebrates and the work has led to the discovery of a 
series of new and interesting chemical compounds. It is now 
intended to discuss the " phosphagen story " in some detail. 

(1) Phosphagens 

In many of the textbooks on comparative biochemistry or 
physiology such as those of Baldwin (1940) or Prosser (1952) one 
will find the following table. 





Arginine Creatine 


Phylum and Class 


phosphate phosphate 


Platyhelminthia 


+ 


Annelida 


+ 


Arthropoda 


+ 


Mollusca : 




Lamellibranchiata 


+ 


Cephalopoda 


+ 



112 



BIOCHEMICAL STUDIES OF PHYLOGENY 113 



Echinodermata : 






Asteroidea 


+ 


— 


Holothuroidea 


+ 


— 


Echinoidea 


+ 


+ 


Protochordata : 






Tunicata 


+ 


— 


Enteropneusta 


+ 


-f 


Cephalochorda 


— 


+ 


Vertebrata 


— 


+ 



This table indicates that most of the invertebrates have one type 
of phosphagen (arginine phosphate) whilst the vertebrates have 
another (creatine phosphate). The echinoderms and the proto- 
chordates have both types of phosphagen and this makes it seem 
likely that they are the group of invertebrates most closely related 
to the vertebrates. 

Let us now consider the situation in a little more detail. 

In 1927 Eggleton and Eggleton showed that one could extract 
a labile organic phosphorus-containing compound from vertebrate 
muscle. This compound was called phosphagen and later workers 
showed that it was in fact creatine phosphate. After isolation crea- 
tine phosphate broke down to form creatine and phosphoric acid. 

NH.OP(OH)., 

/ 
HN = = C + H 2 

N— CH 2 .COOH 



CH 3 

Creatine phosphate. 

NH 2 

HN = C + H 3 P0 4 

N— CH 2 .COOH 

CH 3 

Creatine and phosphoric acid. 



114 



BIOCHEMICAL STUDIES OF PHYLOGENY 



Creatine phosphate (CP) was present in many vertebrate muscles, 
thus the Eggletons found it in the muscles of Amphioxus, dogfish, 
plaice, frog, snake, tortoise, rabbit and guinea-pig. They were 
unable to find it in any of the invertebrate muscle they studied 
(Aurelia, Lumbricus, Aplysia, Pecten, Holothuria). Meyerhof 
(1928) found that there was a phosphagen present in invertebrate 
muscles but that it was not creatine phosphate but arginine 
phosphate instead. This he found in Sipunculus, Pecten, Holothuria 
and Stichopus muscle. 



NH.OP(OH) 2 



NR 



HN = C 



HN = C 



NH 



NH 



(CH 2 ) 3 + H 2 



(CH 2 ) 3 + H 3 P0 4 



CH.NH 2 

COOH 

Arginine phosphate 



CH.NrL 



COOH 



Arginine and phos- 
phoric acid. 



Although it is not strictly within the scope of this book it might 
be as well to indicate the function of the phosphagens. They act 
as an energy reserve for muscle contraction. The phosphagens are 
high-energy compounds and they can phosphorylate adenosine 
diphosphate (ADP) to form adenosine triphosphate (ATP). 

ADP + CP = ATP + C 

When a muscle contracts and performs work, at some stage of the 
contraction-relaxation cycle it uses up the ATP and converts it 
to ADP. This ADP is reconverted to ATP by means of the 
phosphagen. At a later stage, glycolysis (the breakdown of glucose 
to carbon dioxide and water) brings about the synthesis of more 
high-energy compounds and the phosphagen is re-formed. This 
reaction CP + ADP = C + ATP is sometimes called the 
Lohmann reaction and the reader can find more details in most 



BIOCHEMICAL STUDIES OF PHYLOGENY 115 

texts on biochemistry (Harper 1959; Baldwin 1957; Fruton and 
Simmons 1958). 

A thorough survey of the distribution of phosphagens CP and 
AP in the animal kingdom was published in 1931 by Needham, 
Needham, Baldwin and Yudkin. In some cases they dissected 
out the muscle tissue from the lower animals ; in other cases they 
used the whole animal, the method used depending upon the 
size and availability of the raw material. 

It should be remembered in all the discussions of their experi- 
mental work that most of the workers were pioneers in the field 
and that present-day criticism of techniques is in no way meant to 
be disparaging. It is only too easy to look back over a quarter of a 
century of research and, being wise after the event, to point out 
the various faults and errors. It is inevitable in a scientific subject 
that the years will bring great improvements in techniques which 
will then indicate that the previously used methods and con- 
clusions were not sufficiently justified. There is but one way of 
making sure that one's work will never contain any errors and 
that is to do no work. 

The technique that Needham et al. used for their analysis of the 
phosphagen was as follows. They cooled their material and dis- 
sected out the required part. This was then weighed, ground up 
with trichloracetic acid, left for 10 min in the cold and then 
filtered. The filtrate was neutralised with NaOH and then CaCl 2 
was added to precipitate the inorganic phosphate. This pre- 
cipitate of insoluble calcium phosphate was spun down in a 
centrifuge and separated from the supernatant fluid. The 
precipitate was dissolved in a few drops of concentrated sulphuric 
acid and the inorganic phosphate then determined. 

The organic phosphate was still in the supernatant solution and 
it might contain the two possible phosphagens, creatine phosphate 
and arginine phosphate (CP and AP). These were analysed as 
follows. If one places CP (or AP) in acid solution, it hydrolyses 
to form either creatine (or arginine) and phosphoric acid. If 
molybdate ions are present the CP hydrolyses much more rapidly 
than does AP. Thus the determination of phosphate after 15 min 
hydrolysis gave an indication of the CP value whilst estimation 
after 15 hr gave both CP and AP values. The value of AP could 
then be determined by subtraction. 

9— IOE 



116 BIOCHEMICAL STUDIES OF PHYLOGENY 

The results from Needham et al.'s experiments are often 
summarised as in the table on page 112, but I should like to present 
them here in slightly more detail. (See also the table on p. 117.) 

(a) Coelenterates 

In Anthea rustica 5-07 g of tentacles gave 0-053 mg of total 
phosphate of which 0-04 mg was due to inorganic phosphate. 
An experiment on 1-05 g of body wall gave a total phosphate of 
0-032 mg and an inorganic level of 0-032 mg. A further experi- 
ment on 2-71 g of tentacle from Anthea cereus gave total phosphate 
of 0-135 mg and an inorganic level of 0-135 mg. The general 
conclusion from these experiments was that the level of phos- 
phagens in anemones was too low to be detected. 

In the ctenophore Pleurobrachia pileus, 33-67 g of total body 
gave a total phosphate of 0-12 mg whilst the inorganic phosphate 
came to 0-069 mg. This gave 42% organic phosphate which could 
be due to phosphagen. 

(b) Platyhelminthes 

0-4 g of Planaria vitta gave 0-056 mg of total phosphate of 
which 0-042 mg was due to inorganic phosphate. This gave a 
value of 24% AP. 0-54 g of Polycelis nigra had a total phosphate 
value of 0098 mg and an inorganic phosphate value of 0-084 mg. 
Hence the value of AP was 14%. 

(c) Nemertines 

2-1 g of the whole body of Lineus longissimus gave a total 
phosphate of 0-987 mg whilst the inorganic phosphate came to 
0-47 mg. This gave a value of 52% for AP. A second reading 
taking 1-45 g of body gave a total phosphate of 0-299 mg and an 
inorganic phosphate of 0-245 mg. The AP value came to 18%. 

(d) Annelids 

Three annelids were analysed, Nereis, Sabellaria and Spiro- 
graphs. Two experiments were carried out on Sabellaria aheolata. 
In one case 1-38 g of whole body were taken which gave a total 
phosphate of 0-505 mg, 24% of which was due to phosphagen. 
In the other case 2-45 g of body gave a value of 0-789 mg of 
total phosphate of which 30% was due to phosphagen. 



BIOCHEMICAL STUDIES OF PHYLOGENY 



117 



In Spirogr aphis brevispira 2-83 g of the body gave 0-768 mg 
of total phosphate of which 63% was due to phosphagen. Another 
estimation from 3-08 g of body gave 0-925 mg of total phosphate 
of which 67% was due to phosphagen. 



Table 1. Selected from information in Needham, Needham, 
Baldwin and Yudkin (1931) showing the amounts of inorganic 
and organic phosphorus-containing compounds in various 

invertebrates. 













PHOS- 






WEIGHT 




INOR- 


PHAGEN 


GROUP 


ANIMAL 


OF 


TOTAL P 


GANIC 


AS % OF 






TISSUE 




P 


TOTAL P 


Coelenterata 


Anthea rustica 












tentacle 


5-07 


0-053 


0-04 


? 




body wall 


1-05 


0032 


0-032 







Anthea cereus 












tentacle 


2-71 


0135 


0-135 




Ctenophora 


Pleurobrachia pileus 


33-67 


0-120 


0-069 


42-0 


Platyhelminthes 


Planaria vitta 


0-40 


0-056 


0-042 


24-8 




Polycelis nigra 


0-54 


0-098 


0-084 


14-8 


Nemertina 


Linens longissimus 


2-10 


0-978 


0-470 


52-5 


Annelida 


Sabellaria alveolata 
Spirographis brevis- 


1-38 


0-505 


0-383 


24-1 




pira 


2-83 


0-768 


0-281 


63-5 




Nereis diversicolor 


3-45 


1-359 


0-781 


50-3 


Sipunculoidea 


Sipunculus nudus 


1-46 


0-660 


0-195 


71-0 


Cephalopoda 


Sepia officinalis 












fin muscle 


1-25 


0-935 


0-79 


15-3 




mantle 


1-19 


2-73 


2-50 


8-3 




Octopus vulgaris 


1-66 


1-77 


1-42 


12-7 


Echinodermata 


Cucumaria planci 


0-49 


0-037 


0-037 


0-0 




Synapta inhoerens 


0-90 


0-425 


0-315 


25-9 




Strongylocentrotus 












lividus 


1-63 


0-247 


0-020 


92-0 




Asterias glacialis 


3-84 


0-308 


0-072 


76-2 


Protochordata 


Balanoglossus 












salmoneus 


0-36 


0-101 


0-057 


42-8 




Ascidia mentula 


7-03 


0-05 


0-039 


22-5 



In Nereis diversicolor seven normal animals were taken in which 
the phosphagen ranged from 15% to 81% of the total phosphate. 
It is of interest that in these measurements the authors found that 



118 BIOCHEMICAL STUDIES OF PHYLOGENY 

whilst for the other annelids mentioned the phosphagen was 
always AP, in Nereis there appeared to be quite a lot of CP in 
five out of the seven normal samples. Thus out of the 15-81% 
due to total phosphagen the amount due to CP was 5-57%. The 
authors considered that these values of CP were due to errors in 
their technique and that creatine phosphate was not actually present. 
In Sipunculus nudus one measurement was made from 1-46 g 
of body wall. This gave a total phosphate level of 0*66 mg of 
which 71 % was due to AP. 

(e) Cephalopoda 

In Sepia officinalis various parts of the animals were analysed 
for phosphagen with the following results : 

Expt. 1 Expt. 2 Expt. 3 



Fin muscle 


4% 


15% 


7% 


Mantle 


13% 


8% 


0% 


Funnel 


6% 


10% 


0% 


Tentacle 


12% 


5% 


0% 



In Octopus vulgaris the values from one animal came to, mantle 
33%, funnel 27% and tentacle 12% phosphagen. It is of interest 
that in both Octopus and Sepia some of the phosphagen was 
apparently due to CP. In five out of the nine cases where phos- 
phagen was present in Sepia there were traces of CP present, in 
one case it being one-third of the total 6% due to phosphagen. 
In Octopus in one case out of the three CP was present, it making 
up half of the total 12% due to phosphagen. 

(f) Echinoderms 

Two measurements on the body wall of Cucumaria planci 
showed no phosphagens to be present. However, two other 
experiments on the phosphagens of the body wall of another 
species of holothuriam, Synapta inhoerens, gave values of 5% and 
25% of the total phosphate content as being due to phosphagen. 
The phosphagen was AP. Nine experiments on the jaw muscles 
from the Aristotle's lantern of Strongylocentrotus lividus gave values 
from 42%-92% of the total phosphate as being due to phosphagen. 
Analysis showed that about one-third or more of this phosphagen 
was due to CP. 



BIOCHEMICAL STUDIES OF PHYLOGENY 119 

In the spines and muscles of Echinocardium cordatum it was not 
possible to detect any phosphagens at all. 

In the tube feet of the starfish Asterias glacialis there was only 
AP present; this made up 76% and 73% of the total phosphate. 

(g) Protochordates 

In Balanoglossus salmoneus three readings were taken from 
different parts of the body. These showed a range in values of 
phosphagen of 14%, 16% and 42% of the total phosphate. In 
the case where 42% of the phosphate was due to phosphagen it 
was shown that 21% was due to CP whilst the other 21% was due 
to AP. In the 14% of phosphagen the value was all due to CP 
whilst the other case of 16% was all due to AP. 

The values given for Ascidia mentula were given as 22% and 
12% of the total phosphate being due to phosphagen (AP). But 
as the authors pointed out, from 7 g of tissue they obtained only 
0-05 mg of total phosphate and 0-039 mg of inorganic phosphate, 
hence the results were not very reliable. 

On page 289 of their paper, Needham et al. drew up the 
following table. 

The animal kingdom could be subdivided into animals that had 

(1) No arginine phosphate: 

Coelenterata Anthea 

(2) Only AP: 

Coelenterata Pleurobrachia 

Platyhelminthia Planaria, Polycelis 

Nemertini Lineus 

Annelida Sabellaria, Spiro- 
graphs, Nereis 

Cephalopoda Sepia, Octopus 

Echinodermata Synapta, Asterias 

Urochorda Ascidia 

(3) CPandAP: 

Echinodermata Strongylocentrotus 
Hemichorda Balanoglossus 

(4) CPonly: 

Cephalochorda Amphioxus 
Craniata Many species 



120 BIOCHEMICAL STUDIES OF PHYLOGENY 

They concluded, " If any evolutionary significance may be 
attached to these findings, it is probable that they support the 
Echinoderm-Enteropneust (Balanoglossus) theory of vertebrate 
descent rather than any of the other views which from time to 
time have been put forward on this question." 

In fact the situation was not quite so simple. Thus though the 
majority of invertebrates have arginine phosphate and the 
vertebrates have creatine phosphate, the authors found creatine 
phosphate in the Annelida {Nereis) and in the Mollusca {Sepia) as 
well as in the echinodermata {Strongylocentrotus) and Proto- 
chordata {Balanoglossus). In Balanoglossus it was present in only 
two of the three specimens analysed. 

It would seem that there are no very good grounds for con- 
cluding from the phosphagen evidence alone that the echinoderms 
and the protochordates are more closely related to the vertebrates 
than are, say, the Annelida or the Mollusca. 

In 1936 Baldwin and Needham repeated some of the determina- 
tions of phosphagens in the echinoderms. There were two 
problems in which they were interested; the first concerned the 
formation of the phosphagen. There should be an enzyme present 
in the tissue that would bring about the phosphorylation of the 
nitrogenous base and Baldwin and Needham decided to investigate 
the properties of this enzyme. Secondly they were not sure about 
the nature of the nitrogenous base in the phosphagen. Though 
they had felt it might be arginine and/or creatine their tests for 
these compounds were not specific tests but general ones. Thus 
the Sakaguchi test for arginine (make the solution alkaline with 
NaOH; add a little a-naphthol, then add a drop of sodium 
hypochlorite solution — a bright red colour develops) is not really 
specific for arginine but is given by the radical marked by a ring. 

NH 2 

\ 

HN=C 

/ 
Fatty acid group *N-CH 2 .COOH 

CH 3 Creatine 




BIOCHEMICAL STUDIES OF PHYLOGENY 121 

It will not react with creatine since creatine has a CH 3 group 
substituted for the H on the N marked with a *. However, other 
compounds such as glycocyamine (this is creatine without the 
CH 3 group and with an H instead) will give a positive reaction. 

NH 2 

\ 
HN=C 

/ 
NH.CH 2 .COOH 

Glycocyamine 

The creatine was determined by the Jaffe reaction, which again 
is not an absolutely specific test for creatine. 

The arginine from the phosphagen in the echinoderm muscle 
was thus tested by the Sakaguchi test, by seeing if the muscle 
extract could synthesise a phosphagen from arginine and phos- 
phate, and thirdly by adding arginase and seeing if urea was given 
off. 

The jaw muscles from Sphaer echinus granulans were shown to 
be capable of synthesising AP (26% of the added phosphoric acid 
being converted) and CP (12% of the added phosphoric acid 
being converted). 

Extracts from the longitudinal muscles of Holothuria tubulosa 
gave extracts that could synthesise AP from A and P (though it 
should be noted that the Russian workers Verbinskaya, Borsuk 
and Kreps (1935) found AP and CP in the muscles of the 
holothurian Cucumaria frondosa). 

Xeedham and Baldwin also quoted work done by P. Baldwin 
on the ophiuorid Ophioderma longicauda and the crinoid Antedon 
mediterranea. Ophioderma had CP whilst Antedon had AP. 

The conclusion the authors drew from their work was that the 
ophiuroids and echinoids and possibly the holothurians had AP 
and CP and the other echinoderms (asteroids, crinoids) had only 
AP. 

Let us see to what use this information is put. Hyman (1950) 
in her text on the echinoderms states on p. 700, " Further bio- 
chemical evidence supporting the close relationship of echinoids 
and ophiuroids concerns phosphagens, or phosphorus carriers, of 



122 BIOCHEMICAL STUDIES OF PHYLOGENY 

great importance in metabolic processes. . . . Crinoids, holo- 
thurians and asteroids have arginine as the phosphorus carrier 
whereas creatine serves this function in the ophiuroids and the 
echinoids (echinoids also have phosphoarginine). Phospho- 
creatine is also characteristic of vertebrates ; creatine in organisms 
results from the methylation of glycocyamine and only echinoids 
and ophiuroids have the enzymes (methylases) necessary for 
performing this reaction. The author is of the opinion that the 
closer relationship of ophiuroids to echinoids rather than to 
asteroids, as usually supposed, is not to be doubted and therefore 
the union of asteroids and ophiuroids into one group is not 
admissible. Further the arrangement recently adopted by 
palaeontologists according to which the asteroids and ophiuroids 
derive from a common somasteroid ancestor and hence are to 
be united into one class Stellasteroidea must somehow be wrong." 

As the reader will have noticed in the analysis of Needham et al. 
(1932) of the phosphagens in the annelids, they found some CP 
as well as AP present in Nereis. They decided that this was possibly 
due to some fault in their technique and that really only the AP 
was present. 

The problem was reinvestigated by Baldwin and Yudkin in 
1949. They used similar techniques to the previous ones; thus 
they differentiated between CP and AP by the rate of hydrolysis 
in molybdate solution and they also used the Sakaguchi and the 
Vosges-Proskauer test for the amino-acids arginine and creatine, 
though neither of these tests, as they pointed out, were absolutely 
specific. 

Twenty-four different species of polychaetes were tested and in 
addition they examined Lumbricus, Phascolosoma and Sipunculus. 

The polychaetes are listed below. 

Amphitrite johnstoni Lepidometria commensalis 

Amphitrite ornata Lumbrinereis sp. 

Arabella iricolor Maldane urceolata 

Arenicola marina Neanthes virens 

Branchiomma vesiculosum Nereis cultrifera 

Chaetopterus variopedatus Nereis diversicolor 

Cirratulus grandis Orbinia ornata 



BIOCHEMICAL STUDIES OF PHYLOGENY 123 

Cistenides gouldii Pista palmata 

Clymenella torquata Sabella pavonia 

Diopatra caprea Sabellaria aheolata 

Enoplobranchns sanguinea Spirographs brevispira 

Glycera dibranchiata Sthenelais leidyi 

They found that many of these animals had CP and AP. Thus 
Amphitrite, Arenicola, Cirratulus, Clymenella, Enoplobranchus, 
Maldane, Nereis cultrifera, Pista, Sabella, Sabellaria and Spiro- 
graphis had AP but no CP. In Chaetopterus, Diopatra, Glycera, 
Lumbrinereis and Orbinia there was CP but no AP. In Lnmbricus 
there was neither CP nor AP. Phascolosoma and Sipunculus had 
only AP. The other polychaetes had AP and CP. 

The first impression of Baldwin and Yudkin (1948) was that 
there was a correlation between the occurrence of AP and the 
sedentary habit since Amphitrite, Sabella, Sabellaria and Spiro- 
graphs all had AP and were sedentary, but further investigation 
(1949) showed that there was no such correlation. Thus even 
closely related genera had different phosphagens: Neanthes virens 
(which used to be called Nereis virens) had both CP and AP whilst 
Nereis diversicolor had only AP. 

There was some doubt whether CP was really creatine phosphate 
and AP, arginine phosphate. Thus though CP on hydrolysis gave 
a positive Vosges-Proskauer test the authors concluded only 
provisionally that it was creatine phosphate and designated it as 
' CP ' and not CP. 

Similarly the arginine phosphate gave a very weak Sakaguchi 
reaction and they doubted if 'AP ' was arginine phosphate. They 
preferred to refer to it as annelid phosphagen — ' AP.' 

The fact that annelids have CP is of importance in deciding 
the phylogenetic importance and significance of the phosphagen. 
Thus previously it was shown that the invertebrates had AP whilst 
the vertebrates, some echinoderms and some protochordates had 
CP. We now see that polychaetes (twelve out of the twenty-four 
tested) had CP. This means that either the presence of CP is not 
a very good phylogenetic indicator or else that the annelids are 
more closelv related to the echinoderms and the vertebrates than 
they are to, say, the molluscs. 



124 BIOCHEMICAL STUDIES OF PHYLOGENY 

Baldwin and Yudkin (1949) also carried out some analyses of 
the phosphagens in echinoderms and protochordates. They con- 
cluded that the hemichordates and the echinoids were unique in 
that they both had CP and AP whilst the other echinoderms 
(except for ophiuroids) had only AP. These results were presented 
in tabular form and indicated that the vertebrates were derived 
from the Echinoderm-Protochordate line. 

In fact the evidence for the phylogenetic value of phosphagens 
is not very good. Out of the three hemichordates studied, 
Balanoglossus salmoneus, Saccoglossus kowalezvsky and Saccoglossus 
horsti, only the former has AP whilst the others have CP. Rees 
(1958) states that in his analysis of twenty specimens of Balano- 
glossus clavigerus he was able to find only CP; there was no 
indication of AP. 

Amongst the echinoids the jaw muscles of Arbacia punctulata 
have only AP and no CP whilst Strongylocentrotus lividus and 
Echinus esculentus have both CP and AP. Similarly Griffiths, 
Morrison and Ennor (1957) showed that though some echinoids 
such as Heliocidaris erythrogramma had both CP and AP, others 
such as Centrostephanus rodgersi had AP and no CP. These 
authors concluded, " The general assumption of Baldwin and 
Needham that both AP and CP are found in the echinoids is thus 
disproved and the results emphasise the necessity for examining a 
number of species within a class before concluding that a particular 
phosphagen is characteristic of the class." If one was to take the 
possession of the phosphagens as a serious phylogenetic feature 
one might conclude that since the ophiuroids have only CP like 
the vertebrates that they in fact are the closest of the echinoderms 
to the vertebrates. 

The phosphagen story took a new turn in the 1950s when 
chromatographic analysis was applied to the guanidine com- 
pounds. As we have already seen, the previous workers were only 
concerned with two guanidines, creatine and arginine, and they 
differentiated these on the rate of hydrolysis and various non- 
specific tests. French workers at the Laboratory of Comparative 
Biochemistry of the College de France (van Thoai, Roche, Robin 
and Thiem, 1953) showed that the annelids contained at least two 
other phosphagens. They found these by running ascending 



BIOCHEMICAL STUDIES OF PHYLOGENY 



125 



chromatograms of muscle extracts from Arenicola marina and 
Nereis dtiersicolor in either pyricline-water or propanol-acetic- 
water and developing the chromatograms in a-naphthol hypo- 
bromite which gives a coloured spot with guanidines. They 
found that in Arenicola there was the compound taurocyamine 
phosphate whilst in Nereis there was glycocyamine phosphate. 



NH, 



NH< 



HN = C 



NH 



NH.CH 2 .S0 3 H 

Taurocyamine 



NH.CH 2 .COOH 

Glycocyamine 



NH— PO(OH) 2 



NH— PO(OH), 



HN 



HN = C 



NH.CH 2 .S0 3 H 

Taurocyamine phosphate 



NH.CH 2 .COOH 

Glycocyamine 
phosphate 



The relationship of these compounds to the other phosphagens 
was not as obscure as might be supposed on first sight. Thus van 
Thoai and Robin (1951) had shown that an enzyme capable of 
methylating various compounds had quite a wide distribution in 
the invertebrates and that it was quite possible that this might 
methylate glycocyamine to form creatine. 

It is probable that glycocyamine phosphate (GP) and tauro- 
cyamine phosphate (TP) play a similar role in the body to AP 
and CP since there are enzymes that can phosphorylate G and T. 
Thus Hobson and Rees (1957) showed that specific phosphokinases 
were present in various annelids. The unphosphorylated base was 
added to the muscle extract, inorganic phosphate and the appro- 
priate buffer. This was then incubated at 40 °C for 15 min and 
the phosphagens formed isolated and tested. The results are 
shown in the following table. 



126 



BIOCHEMICAL STUDIES OF PHYLOGENY 

Table 2 





|UM 


OF PHOSPHATE FORMED 


ANIMALS 












TP 


GP 


AP 


CP 


Arenicola marina 


5-0 


1-25 








Nereis diversicolor 


0-4 


6-5 








Nereis facata 





5-8 





20 


Hermione hystrix 











4-5 


Aphrodite acideata 











1-25 


Myxicola infundibulum 


1-5 











Nephthys cacea 





1-7 





0-7 



From Table 2 it can be seen that Arenicola and Myxicola have 
TP whilst Nereis diversicolor has GP as its main phosphagen. 
Hermione and Aphrodite have CP whilst none of them had AP. 

About the same time Roche and Robin (1954) showed the 
presence of CP in the sponge Thetia lyncurium and AP in the 
sponge Hymeniacidon caruncula. Hymeniacidon also had glyco- 
cyamine (but not GP) whilst Thetia had taurocyamine (but no 
TP). 

Roche et al. (1957) made a thorough survey of the distribution 
of arginine and creatine in the animal kingdom, using the methods 
of chromatographic separation and semi-specific chemical tests. 
The species that they analysed and the results they obtained are 
shown below. 



Species 


Creatine 


Arginine 


Sphaerechinus granulans 


+ 


+ 


Martasterias glacialis 


+ 


+ 


Amphipholis squamata 


+ 


+ 


Ophiothrix fragilis 


+ 


+ 


Leptosynapta inhoerens 


+ 


+ 


Maia squinado 





+ 


Apis mellifica 





+ 


Bombyx mori 





+ 


Sepia officinalis 





+ 


Helix pomatia 





+ 


Limnaea stagnalis 





+ 


Mytilus edulis 





+ 


Ostrea edulis 





+ 



BIOCHEMICAL STUDIES OF PHYLOGENY 127 

Species Creatine Arginine 

Arenicola marina 
Audouinia tentacidata 
Clymene himbricoides 
Dasybranchus caducus 
Glycera convoluta 
Lineus marinus 
Lnmbriconereis 
Marphysa sanguinea 
Nephthys hombergi 
Nereis diversicolor 
Sabella pavonina 
Scolophus armiger 
Lumbricas terrestris 
Hirudo medicinalis 
Phascolosoma elongatum 
Sipuncidus nudus 
Ascaris himbricoides 
Actinia equina 
Anemonia sulcata 
Calliactis parasitica 
Halicho?idria panicea 
Hymeniacidon caruncula 
Thetia lyncurium 
Tetrahymena geleii 

The authors conclude that the very wide distribution of 
creatine does not allow one to come to any conclusion concerning 
its phylogenetic importance and in particular the presence of 
creatine in the echinoderms in no way indicates an affinity or 
relationship with the vertebrates. This view is supported by 
Ennor and Morrison (1958) in their review of the biochemistry of 
phosphagens and related guanidines. 

What conclusion can be drawn with regard to the distribution 
of phosphagens in the animal kingdom? The first conclusion is 
that there is certainly no simple cleavage of the animal kingdom 
into vertebrates with CP and invertebrates with AP. Instead it is 
clear that both CP and AP are found throughout the invertebrates. 
The second conclusion is that one cannot base any phylogenetic 



+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 





+ 


+ 


+ 


+ 


+ 





+ 





+ 





+ 


+ 


+ 





+ 





+ 





+ 


+ 


+ 





-f 





+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 


+ 





+ 



128 BIOCHEMICAL STUDIES OF PHYLOGENY 

speculation on the occurrence of CP or AP since related genera 
within a class can differ widely in their phosphagens. The third 
conclusion is that it is highly probable that other phosphagens in 
addition to the recently discovered GP and TP will be found to 
play a role in tissue metabolism. 

Thus Seaman (1952) has isolated a phosphagen from Tetra- 
hymena gelei that does not appear to be any obvious guanidine 
derivative, i.e. not arginine, creatine, taurocyamine or glyco- 
cyamine. Van Thoai and Robin (1954) isolated another phosphagen 
from the muscles of Lumbricus and called it lombricine. Its 
structure is shown below. Lombricine has been further analysed 
by Beatty et al. (1959), who have shown that it contains a d- 
amino-acid, D-serine. 

NH 2 NH 2 



C = NH O CH— COOH 

I II I 

HN— CH 2 — CH 2 P O— CH 2 

OH 

Lombricine 

Robin, van Thoai and Pradel (1957) described a new guanidine 
derivative in the leech Hirudo medicinalis but have not as yet 
published details of its structural formula. 

The function of a scientific theory is to help in our understand- 
ing of various pieces of information and to suggest further experi- 
ments that will test the validity of the theory. The value of a 
theory lies in the extent to which it stimulates the development of 
such new experiments. The theory concerning the distribution of 
phosphagens throughout the animal kingdom as suggested by 
Baldwin and Needham has been a very valuable one when judged 
in this manner. 

It is becoming clear that the initial impetus that Baldwin, 
Needham et al. gave to the study of phosphagens has gathered 
momentum and a great deal of new information has been gathered 
concerning the chemical nature of phosphagens. What is now 
required is a very thorough analysis of material from many genera 
throughout the whole of the invertebrates for information as to 



BIOCHEMICAL STUDIES OF PHYLOGENY 129 

the variety of phosphagens. It is certain that many new chemicals 
still remain to be discovered and the biochemical variations will 
probably be found to be as great as the more obvious morphological 
variations. 

From the phylogenetic point of view, therefore, the phosphagens 
are not a great deal of assistance. The table shown on p. 112 
can now be amended as below. 









Other 


Phylum 


AP 


CP 


guanidines 


Protozoa 


+ 




+ 


Sponges 


+ 


+ 


+ 


Coelenterata 


+ 


+ 


+ 


Platyhelminthia 


+ 






Nemertina 


+ 






Nematoda 


+ 






Annelida 


+ 


+ 


+ 


Arthropoda 


+ 






Mollusca 


+ 


+ 




Echinodermata 


+ 


+ 




Protochordata 


+ 


+ 




Vertebrata 




+ 





The gaps in the table will be filled as more research is done on 
this subject. 

(2) Sterols 

The phosphagens are not the only compounds that have been 
used to indicate phylogenetic relationships. Within recent years 
certain sterols have been used to elucidate relationships, though the 
work is still at a developmental stage. Some applications of the 
sterol studies are as follows. 

Hyman (1955) in her volume on the echinoderms does not 
follow the normal custom and place the asteroids with the 
ophiuroids ; instead she places her chapters in the order Holothuria, 
Asteroidea, Echinoidea, Ophiuroidea. Her reasons for doing this 
are stated on p. 699. In particular she decides that the ophiuroids 
and the asteroids should be separated and the ophiuroids placed 
withtheechinoidson the basis of larval development, the possession 
of an epineural canal in the ophiuroids, and the possible occurrence 



130 




BIOCHEMICAL STUDIES OF PHYLOGENY 

CH 3 

CH— CH 2 .CH 2 R 




Cholesterol 



Cholestanol 



CH 3 

CH— CH 2 CH 2 R 



CH 3 
CH.CH 2 .CH 2 .R 




CH 3 
CH.CH=CH.R 




Clionasterol 



Porifasterol 



CH 3 

CH — CH=CH.R 




Stellasterol 

Fig. 39. Sterol structure. This figure shows the structure of various 
of the sterols mentioned in the text. 

R = — CH 2 .CH(CH 3 )<, for Cholesterol and Cholestanol. 
R = -CH.(CH 3 ).CH"(CH 3 ) 2 for the others. 

of a vestibule in the ophiuroids. On p. 700 she states, " Finally, in 
recent years workers in comparative biochemistry have produced 
striking evidence in favour of this community of ancestry " (i.e. 
the ophiuroids with the echinoids and not with the asteroids). 
" Bergman (1949 and in a letter) finds that all ophiuroids and 
echinoids tested have sterols of Type I, namely cholesterol or some 
closely related compound, whereas numerous asteroids tested have 
Type II sterols that is, stellasterol or related compounds. The 



BIOCHEMICAL STUDIES OF PHYLOGENY 131 

sterols of the three crinoids thus far tested belong to Type I, al- 
though perhaps a new variety, and those of holothurians classify as 
Type II. Further biochemical evidence supporting the close re- 
lationship of echinoids and ophiuroids concerns phosphagens or 
phosphorus carriers, of great importance in metabolic processes." 
Bergman (1949) gives the following table showing the various 
species of echinoderms studied and the sterols present in each 
(see Fig. 39). 

Asteroidea Stellasterol Hitodestrol Cholesterol 

Asterias rubens * 

Asterias forbesi * 

Asterias rollestoni * 

Asterias scoparius * 

Asterias pectinifera * 

Echinoidea 

Tripneustes esculentus 
Centrechinus antillaram 
Lytechinus variegatus 
Heliocidenis crassidus 
Arbacia punctulata 

Holothuria 
Holothuria princepo * 

Cucumaria chronjhelmi * 

Ophiuroidea 
Ophiopholis aculeata # ? 

From this table one can se e that the asteroids and the holothur- 
ians both possess stellas te rol whilst the echinoids and possibly the 
ophiuroids have cholesterol. This would link the asteroids and the 
holothurians on the ne hand and the echinoids and ophiuroids 
on the other hand, an arrangement which would agree with that 
based on larval ch aracte ristics. 

Perhaps it will pay us to look at the steroid situation in a little 
more detail. Bergman (1949) has given an interesting review of the 
distribution of lipids in marine invertebrates with special refer- 
ence to the sterols. At one time it was thought that cholesterol was 
the only sterol present in these bodies but later work showed that 

10— IOE 






132 BIOCHEMICAL STUDIES OF PHYLOGENY 

there were in fact over twenty different sterols, including optical 
isomers, present in the invertebrates and that it is quite likely as 
research proceeds that still more will be discovered. These 
sterols differ in (1) the length of the side chain, (2) the presence or 
absence of double bonds in this side chain, (3) the location of 
double bonds in the main sterol skeleton. 

Up to 1949 the most studied phylum was the Porifera. This was 
due to the fact that they were easily obtained in fairly large 
quantities. Over fifty different species of sponges have been 
analysed by Bergman and his colleagues and they have obtained 
some very interesting results. In the first place they have dis- 
covered more than ten different sterols in sponges, only two of 
which had been known before. Secondly the presence or absence 
of these sterols helped in the elucidation of certain systematic 
problems. 

For some time a sponge from the Biscayne Bay, Florida, had 
been given a variety of names. Some collectors had called it 
Suberites distortus, others Suberites tuber culosus. Bergman (1949) 
studied the sterols present in this sponge and showed that 
clionasterol and poriferasterol were present. Now these sterols 
were normally not found in the family Suberitidea but instead 
were more often found in the Clionidae or the Choanatidea. 

Porifer- Neospongo- 
sterol 





Cholest- 


Clionast- 


Porifi 




anol 


erol 


aster 


Choanitidae 




# 


# 


Suberitidae 


# 






Clionidae 




# 


# 


Suberites distortus 




* 


# 



# 



Suberites was very carefully examined by Laubenfells, who 
showed that there were some small microscleres present in the 
tissues. These microscleres were diagnostic of the genus 
Anthosigmelia, which is in the family Choanitidea. This then would 
mean that the sponge was not a Suberitidae but a Choanitidae and 
this would agree with the sterol assay. 

A more complex case is present in that of Hymeniacidon 
heliophila. This sponge has been described both as Hymeniacidon 
heliophila and as Stylotella heliophila. The genus Stylotella is 



BIOCHEMICAL STUDIES OF PHYLOGENY 133 

normally placed in the family Suberitidae, a family containing only 
saturated sterols. Bergman has shown that saturated sterols 
are present in this sponge and therefore the sterol assay agrees 
with its placing amongst the Suberitidae. Unfortunately the range 
of sterols of the family Hymeniacidonidae is not yet known but 
there is no reason why this sponge should be transferred from the 
genus Hymeniacidon to that of Stylotella. Bergman states, " such 
a transfer will remain premature until more is known about the 
sterol contents of other species of Hymeniacidon and of the closely 
related Halichondria " and he makes his position even more clear 
in the statement, " It is dangerous and frequently misleading to 
base significant conclusions concerning comparative biochemistry 
on data derived from but a few representatives of a given phylum. 
This point is apparent when the great diversity of sterols in 
Porifera is considered." 

To return to the echinoderms it will be remembered that so far 
only three types of sterol have been described in the literature 
for the echinoderms. There is a likelihood that further study of 
the echinoderms will show a greater divergence of the sterols 
present in this group and that the cleavage shown in the table on 
p. 131 will not be so clearly delineated. 



CHAPTER 9 



VERTEBRATE PALAEONTOLOGY 



The most important evidence for the theory of Evolution is that 
obtained from the study of palaeontology. Though the study of 
other branches of zoology such as Comparative Anatomy or 
Embryology might lead one to suspect that animals are all inter- 
related, it was the discovery of various fossils and their correct 
placing in relative strata and age that provided the main factual 
basis for the modern view of Evolution. 

It is unfortunate that the earliest rocks to contain fossils, the 
Precambrian and Cambrian, already show representatives of all 
the major invertebrate phyla. The earliest rocks are mainly 
igneous and it is possible that the fossils that they once contained 
have since been boiled away, but there is an alternative view that 
the invertebrates suddenly and explosively evolved and had little 
or no Precambrian history. Though there is some development of 
the various invertebrate fossils, especially within the phyla, our 
main examples of the evolution of the major groups of animals 
come from our study of the vertebrates. If we ask an under- 
graduate to give a brief account of the way in which the vertebrate 
palaeontology provides evidence for evolution, his answer may go 
rather like this. 

" It is possible to date the rocks fairly accurately and in general 
the oldest rocks are at the bottom and the youngest rocks are on 
the top. There are sometimes cases where the rocks have been 
turned over so that the layers are sideways on or upside down, but 
careful study soon indicates this and allows one to determine 
their correct relative positions. If one studies the vertebrate 
remains, one finds that there are no vertebrate fossils in the oldest 
rocks. The next oldest rocks have some vertebrate fossils; these 
are fragments of simple fishes. The next oldest rocks have fish 

134 



VERTEBRATE PALAEONTOLOGY 



135 



and amphibian fossils, the next have fish, amphibian and reptile 
fossils, whilst the most recent rocks will have fish, amphibian, 
reptile and mammal fossils (see Fig. 40)." 

"The most important point is that one never finds a mammal 
fossil in rocks that are pre-reptilian ; in fact the finding of a single 
mammal fossil in such an early stratum would seriously question 
the correctness of evolutionary concepts. Such a fossil has never 
been found and the evidence now accumulating strongly supports 
the view that the fish gave rise to the amphibia, the amphibia to 
the reptiles, and the reptiles to the mammals." 



Coenozoic 



Cretaceous 

Jurassic 

Triassic 



Carboniferous 



Devonian 



Cambrian 



T 



Mammals 



Reptiles 



Amphibia 



Fish 



No vertebrate fossils 



Fig. 40. Diagram to illustrate a simple view of the level of origin of 
the various vertebrate fossils. Note that the sequence runs 

Fish-Amphibia-Reptilia-Mammals. 



This account, though a simple one, contains one serious fault. 
The figure shows not the time of origin of the different classes of 
the vertebrates but instead the time of dominance of that class. If 
we consider the time of origin we get a more complex picture (Fig. 
41). Thus instead of having the reptiles, amphibia, bony fish and 
elasmobranch fishes all separated from each other by hundreds of 
millions of years, they all arose during the course of less than 100 
million years. It is of course difficult to decide just when any of 
the groups did arise, but some estimate can be made. 

The earliest fossil vertebrates, the Agnatha, are found in the 
Silurian (fragments are found in the Ordovician). The next group, 
the Placoderms, are found in the Upper Silurian. The bony fish 
arose in the Devonian as did also the elasmobranches and the 
Amphibia. (It is of interest to note here that there is one school 
of thought, examplified by Save Soderberg (1934) and Jarvik 



136 



VERTEBRATE PALAEONTOLOGY 



Coenozoic 







_ 






Cretaceous 

Jurassic 

Triassic 








Mammals 
1 


Carboniferous 






Reptiles 

■ 


1 

? 


Devonian 


Fish Ar ; phibia ? 
i ■ 


Cambrian 


i ? 

? No vertebrate fossils 



Fig. 4 1 . Diagram to show a more complex view of the level of origin 

of the various vertebrate fossils. Note that the precise time of 

origin is often not clear and that the jawed fish, amphibia and 

reptilia all arose within a comparatively short time of each other. 



(1942), that suggests that the modern amphibia are diphyletic, 
the anurans coming from one stock of bony fish whilst the 
urodeles came from another; the two lines being separate in the 
Early Devonian.) 

The reptiles arose in the Carboniferous. There are certain 
forms such as Seymouria that are of interest in that they have body 
characters that are reptilian and head characters that are amphibian. 
Seymouria is sometimes thought of as a link between the Amphibia 
and reptiles. Unfortunately Seymouria is found in the Permian 
whilst the first reptiles arose in the Pennsylvanian, some 20 or so 
million years earlier. The situation concerning the origin of the 
mammals is not very much more clear, though the mammals 
certainly evolved at a later date than the first reptiles. Just how the 
major groups of the mammals evolved is not very clear. Thus we 
have three distinct mammalian lines, the Monotremes, the 
Marsupials and the Placentals, and there is no good evidence that 
all three came from the same reptilian stock. It is quite possible 
that many of the mammalian-like characters such as warm- 
bloodedness, double circulation through the heart, development 
of the neopallium, development of hair and secretion by milk 
glands, could be homoiologous and that the mammals resemble the 
arthropods in that they are not a phylum but a grade of organisa- 
tion that has been achieved many times from a basic stock. 

There are other points of interest that arise when we consider 
the time of origin of these various groups. On embryological 



VERTEBRATE PALAEONTOLOGY 137 

grounds it had been considered that cartilage was more primitive 
than bone. Thus cartilage appeared in the embryo in most cases 
before bone did, and the elasmobranch fishes in many ways 
appeared simpler than the bony fish. The elasmobranches were 
in fact considered to be more primitive than the bony fish and it 
was not till more attention was paid to the palaeontological dating 
that it become clear that the elasmobranches were more recent 
than the Osteichthyes. The Osteichthyes arose in the Early and 
Middle Devonian whilst the elasmobranches arose in the Middle- 
Late Devonian. Furthermore most of the fossil groups were bony 
when first found but there was a tendency to reduce the ossifica- 
tion so that the later forms are less bony and more cartilaginous. 
The palaeontological evidence thus indicates that bone is more 
primitive than cartilage and in this respect conflicts with ideas 
that are derived from embryological studies. 

The fact that the groups Agnatha, Placoderms, Osteichthyes, 
Chrondrichthyes, Amphibia and Reptiles all arose within a 
relatively short time of each other (possibly by explosive evolution, 
the explosion lasting over 50 million years) means that one has to 
be much more accurate in dating the fossils than if it had taken, 
say, 300 million years. 

There are two main ways of dating rocks: an objective method 
of using radioactive data and a subjective method by which one 
analyses the relative position of the rocks and their included 
fossils and then comes to conclusions concerning the contempor- 
aneity and priority of the different strata. Neither of these 
methods is completely free from objection, as we shall now see. 

Radioactive dating of rocks 

There are various methods by which it is possible to use the 
ratio of various radioactive materials to determine the absolute 
age of rocks. Some of these will briefly be mentioned here. 

The first method is the uranium-lead method, or the so-called 
radiogenic lead method. Nier (1939) published a review of this 
method as applied to various samples and later reviews of the 
subject have been written by Knopf (1948), Kulp (1955, 1956) and 
Ahrens (1956). There is also the book by Zeuner (1958) which 
discusses the various methods of dating material. The common 
lead isotope has the atomic weight of 204, However, other 



138 VERTEBRATE PALAEONTOLOGY 

isotopes of lead occur and these are formed during the breakdown 
of uranium and thorium. Thus 

TJ238 ^ Pb 206 

TJ235 ^ Pb 207 

1^232 y p]^208 

The rate of decay of U 238 — >- Pb 206 is constant and can be deter- 
mined by experimental observation. It is usually expressed as a 
half-life period, i.e. the time taken for x g of uranium to decay 
into x/2 g of uranium. Since the half-life period of the three 
reactions above are known it is possible to get three checks on the 
age of any piece of rock that contains U 235 , U 238 and Th 232 . In 
less than one million years all three reactions come into equilibrium 
and the ratio of the values U 238 /Pb 206 ; U 235 /Pb 207 and Th 232 /Pb 208 
should be constant. The ratio of Pb 207 /Pb 206 should also be con- 
stant since the ratio of U 235 /U 238 is constant. This means that on 
old rocks all three methods should give results of approximately 
the same value. 

This radiogenic dating has been of the greatest value in deter- 
mining the age of the earth. These studies indicate that the 
earth is much older than most people had thought and that it is 
of the order of 4,500 million years old. But when the radiogenic 
methods are applied to more recent rocks, especially those bearing 
fossils, two serious handicaps arise. The first is that this method 
can of course only be applied to rocks that contain radiogenic 
lead; that is, lead derived from uranium or thorium. These rocks 
are usually pegmatites, i.e. rocks formed from the residues after 
granite has crystallised out from the liquid mass. This implies 
that the material at some stage or another has been molten and 
that therefore it is unlikely to contain fossils. Secondly there are 
considerable differences in the age as determined from the differ- 
ent ratio of the isotopes 206/207, 206/238, 207/235 or 208/232. 
Thus Kulp (1955) has published a table giving data for forty-five 
different samples of material, the lead ratios being determined by 
mass spectrometer; and of these only seven are believed to be 
accurate to within 5%. Some are very inaccurate, due, it is 
believed, to the loss of radon by diffusion from the rocks in the 
series U 238 /Pb 206 . Another difficulty is due to the amount of 



VERTEBRATE PALAEONTOLOGY 139 

non-radiogenic lead present in the material. Where this is 
high there is a corresponding high error in the estimation. This 
can lead to an error of 700 million years in the exceptional case 
of the Caribou Mine, Colorado, where the deposit contained as 
much as 97% lead. The correct age of the deposit was 25 million 
years old. 

From the stratigraphic point of view the radiogenic data is a 
little disappointing. Thus Kulp (1955) states, " The only 
thoroughly satisfactory sample from a stratigraphic point of view 
is the Swedish kolm which contains Upper Cambrian fossils." 
Unfortunately the isotopic ages obtained by Nier for this sample 
do not agree. Thus: 

U 238 /Pb 206 gave 380 million years 
Tj235/pb>2 07 gave 440 million years 
Pb 207 /Pb 206 gave 800 million years 

The correct age appears to be 440 million years and it is probable 
that the other values are in error due to radon loss. 

There are two other locations of fossiliferous rocks that have 
also been accurately radiogenically dated. A pitchblende from 
Colorado has been dated as 60 million years old. This had been 
placed at the beginning of the Eocene. It will be remembered 
that the dating of the Eocene was tentatively done by Matthew 
(1914) by estimating the time required for the evolution of the 
horse. Matthew decided that it took 45 million years, i.e. he 
differed by 15 million years from the radiogenic dating. 

All these dates are based on pitchblendes which might have 
percolated through into the examined strata and they could in 
fact have been derived from some other strata than that in which 
they have been discovered. Kulp in discussing the uranium 
contents of rocks states that the high uranium concentration is 
often associated with a carbonaceous deposit and it is conceivable 
that the uranium was accumulated by some biochemical process 
before the rock systems became molten. 

The fact that pegmatites are few and far between makes it 
improbable that the uranium method will have extensive use in 
dating fossils. It is more likely that some other method will be of 
greater use. 



140 VERTEBRATE PALAEONTOLOGY 

Potassium method 

This is one of the most promising new methods for the dating 
of rocks. Potassium is one of the most common elements in rock 
and its isotope K 40 occurs as 0-0119% of the natural element. K 40 
decays to form Ca 40 by beta emission, the decay having a half-life 
period of 1-35 X 10 9 years. This would take us back 1,000 
million years. There is a second path of decay open to K 40 . 
It can capture an electron and turn into A 40 . This latter system 
has not yet been fully worked out but it is probable that both 
methods will prove of great use in dating rock strata (Ahrens 1956). 
A paper by Mayne, Lambert and York (1959) shows that whereas 
the previous methods estimated the Upper Cambrian to be 450 
million years old, the K method gives a value of 650 million 
years. This would mean that these rocks are some 200 million 
years older than previously thought, a point of considerable 
interest since it indicates the size of the errors to be expected in 
estimates based on other methods. 

Subjective Methods 

We have, then, as yet, no accurate objective clock that will 
allow us to determine the absolute age of the majority of the rocks 
of the world. Instead we have to go mainly on stratigraphical 
data and there too we find several problems. 

From a stratigraphical point of view one cannot state the 
absolute age of a given piece of rock ; all that one can do is estimate 
the relative age of the rock, and where this is based on the thick- 
ness of the deposit and the rate of deposition, the results are bound 
to be only approximate. If the various levels are complex and 
stratified, then it is often possible to determine contemporaneity, 
especially when the strata are close together. The situation is 
much more complex when one has to decide if rocks in different 
parts of the world are contemporaneous. Thus the great Caledonian 
Oregony gave rise to a marked separation of the Devonian and 
Silurian rocks in North- West Europe, but this separation is not 
found in the Appalachian syncline. Similarly the rock strata in 
Maryland, U.S.A., show an unbroken series of deposits from the 
Late Silurian to Early Devonian without any sign of a boundary. 

To what extent can one place the fishes found in England, 
Scandinavia, Germany and the United States in their correct 



VERTEBRATE PALAEONTOLOGY 141 

relative temporal positions? To what extent can one assume that 
the climatic conditions in different parts of the world would not 
have affected the distribution of animals so that a local change in 
climatic conditions lasting some millions of years did not lead to a 
migration of specimens into different parts of the world? For 
then, if one decided that strata with similar fossils were of similar 
age, one could be in serious error. What is the maximum error 
that can be allowed in the estimation of the date of any given 
series of rocks? All these questions have to be answered before 
one can decide that fish A is earlier than fish B in time. 

There is another and in this case a minor difficulty. This 
concerns the nature and naming of the strata. There is often some 
discussion as to whether a given series of rocks should be placed 
at the bottom of one level or the top of another. Thus the 
Tremadoc has been placed both at the top of the Cambrian and 
also at the bottom of the Ordovician. Another problem concerns 
the Downtonian; are these Late Silurian or Early Devonian rocks? 
This point is only of importance when the fossils are classified as 
Late Silurian or Early Devonian instead of Downtonian. It is 
indeed a great tribute to the work of the geologists and palaeonto- 
logists that so much agreement has been reached concerning the 
dating of the various strata. But it is unfortunate that the 
difficulties are often glossed over and only the most simple story 
presented. When one is dealing with the evolution of the basic 
vertebrate types all within a comparatively short time of each 
other, the problem of accurate dating becomes one of critical 
importance. Many of the conclusions that we have today are only 
tentative ones. 

We can state with certainty that the earliest bony fragments are 
those of Agnathan fish. These are separated by some 300 million 
years from the earliest mammalian fragments. But it is much more 
difficult to decide just how much earlier the Agnatha were than the 
very first Placoderms; or how much earlier the first Osteichthyes 
were than the first elasmobranches, or when the first Amphibia 
arose relative to the time of origin of the reptiles. We can believe 
that one group arose before the other and there is good evidence 
that one group of fossils may be commonly found before another, 
but when it comes down to giving a precise date, or even a reason- 
able estimation of the time of origin of the groups, it is quite 



142 VERTEBRATE PALAEONTOLOGY 

another matter. Thus we don't know the time or the source of 
origin of the vertebrates. We do not know the relationship 
between the Agnatha and the Placoderms. We do not know the 
ancestry of the Osteichthyes or Chondrichthyes. We do not know 
if the Amphibia are monophyletic or diphyletic. We do not know 
if the mammals are monophyletic or polyphyletic. 

In spite of the ignorance on these basic points certain changes 
have been taking place. Thus in older classifications one could 
read about a group called " Pisces." This group is no longer 
considered a suitable classiflcatory unit and it has been broken 
down into the groups Agnatha, Placoderms, Osteichthyes and so 
on. In other words the group " Pisces " was a complex grade of 
organisation, a grade corresponding to the " fish level of com- 
plexity." Further studies may show that the Amphibia, reptiles 
and mammals are all grades of organisation and not necessarily 
very closely related groups of animals. 

Rates of Evolution 

Within recent years the study of genera and species in the 
various geological strata has been put on a quantitative basis. 
Thus if one studies a group of animals and examines the number of 
genera present, say, in the Ordovician and then examines how 
many of these genera are present in more recent strata one can 
make a calculation of the time over which each genus existed and 
from this one mav come to certain tentative conclusions about the 

J 

evolution of the groups as a whole. 

This technique, together with various others, has been applied 
with considerable success by G. G. Simpson in his books Tempo 
and Mode in Evolution (1944) and The Major Features of Evolution 
(1953). These works satisfy two desires in the reader: the first 
is for an intelligent approach to fossil animals and the realisation 
that they once were living animals; the second is for a treatment of 
evolution and palaeontology in a mathematical and symbolic 
manner. 

Simpson takes two groups, the Lamellibranches and the 
Carnivores (excluding the Pinnipedia) and for each draws up a 
table showing the number of genera present in the Ordovician, 
Silurian, Devonian and so on, and also the level at which each 



VERTEBRATE PALAEONTOLOGY 143 

genus disappeared. From this information it is possible to draw 
a curve showing the percentage of Devonian genera that are alive 
at more recent times. From these curves one can see that the 
lamellibranches differs from the Carnivora in that the mean 
survivorship for a lamellibranch genus is some 78 million years 
whilst that for the Carnivora is 6J million years. Simpson con- 
cludes, " The data undoubtedly exaggerate the difference for 
various reasons, but it is safe to say that the carnivores have 
evolved, on the average, some ten times as fast as pelecypods 
(lamellibranches) " (1944). This does not mean that all lamelli- 
branch genera lasted for 78 million years and hence ten times 
as long as each carnivore genus; the values referred to are mean 
values. 

Simpson's views concerning the mean length of genera have 
not gone unchallenged. In particular Williams (1957) has made 
some interesting objections to the techniques employed by 
Simpson. Williams states, " An ever-increasing number of papers 
dealing with the development of fossil groups contains a host of 
graphical and numerical devices designed to provide a sober tone 
of objectivity to the accompanying text. On the whole they appear 
to be extremely useful but there is a real danger that the student 
will lose sight of the tenuous and arbitrary nature of most of the 
data used in the compilation of such charts, for there is always a 
tendency to accept numbers as the only worthwhile facts in papers 
of this kind." 

Williams goes on to point out that a great deal in such calcula- 
tions depends on the nature of the systematics of the groups studied 
and whether the systematists working on the groups were 
" lumpers " or " splitters." The former group as many species 
as possible together into one genus, the latter separate each species 
into a separate genus! If a lumper has been at work on a group, 
there would be few genera and each would exist for a long period 
of geological time. There are also comparatively few genera if a 
group has not been " monographed " for some time. (There 
appears to be a good correlation between the number of mono- 
graphs that has been published on a group and the number of 
genera described for such a group (Cooper and Williams 1952).) 
When the concept of " lumpers and splitters " is applied to the 
Brachiopods, a group that includes Lingida which has remained 



144 VERTEBRATE PALAEONTOLOGY 

unchanged since the Early Cambrian and which Simpson classifies 
as a slowly evolving group, it becomes clear that different views 
concerning the survivorship of the genera can be obtained by 
examining the genera described in 1894, 1929 or 1956; the mean 
life of a genus being 64, 56 or 53 million years respectively. This 
would still mean little when compared to the 6J million years 
of the carnivore genus, i.e. the brachiopods would appear to have 
evolved some seven times more slowly. 

Williams then points out that the figure for the carnivores is 
that of a small group taken over the climax of their evolutionary 
history, i.e. when the carnivores show the greatest variation and 
formation of new genera. On the other hand the figures for the 
lamellibranches and the brachiopods are taken over the whole 
range of the animals' geological record and both groups are 
found from almost the earliest geological time. Williams therefore 
suggests that it is more logical to consider the climax of evolution 
of such group as the brachiopods. This would be the 231 genera 
of Ordovician times. The average duration of these genera is 16 
million years and some have as short a duration as 10 million years. 
Thus if the climax of evolution is taken for both the Carnivora and 
Brachiopoda, the difference of the mean duration of each genus 
changes from one of 6| million years and 53 million years to one of 
6J million years and 16 million years. It would be interesting to 
have the similar calculations applied to the lamellibranches and, 
say, the early reptiles, the latter showing an explosive type of 
evolution that took place some time ago, thus allowing the post- 
climax period to be analysed. The example just quoted concerning 
the rate of evolution of the Brachiopoda shows how careful one 
must be in assuming that conclusions are valid unless one makes 
a careful consideration and analysis of the data supporting 
these conclusions. 

The Evolution of the Horse 

It would not be fitting in discussing the implications of Evolu- 
tion to leave the evolution of the horse out of the discussion. The 
evolution of the horse provides one of the keystones in the teach- 
ing of evolutionary doctrine, though the actual story depends to a 
large extent upon who is telling it and when the story is being 



VERTEBRATE PALAEONTOLOGY 145 

told. In fact one could easily discuss the evolution of the story of 
the evolution of the horse. 

It started when Kowalevsky in 1874 working with European 
and Asian forms drew up the scheme shown below. 

Equus (Pleistocene- Recent) 

HI 

Hipparion (Pliocene) 

t 

Anchitherium (Miocene) 

t 

Palaeotherium (Eocene) 

These fossil types showed the trends in the evolution of the 
modern horse, i.e. increase in size of the body, reduction in the 
number of digits, molarisation of the premolars, etc, even though 
in fact later workers showed that Palaeotherium, Anchitherium 
and Hipparion were not even on the main line to the evolution of 
the horse. In particular, Kowalevsky was handicapped in studying 
only Old World horses whilst it has been clearly shown by the 
magnificent work of American palaeontologists such as Marsh, 
Cope, Leidy, Osborn and Matthew that the major development 
of the horse took place in the New World. In 1917 Lull published 
a scheme showing the then current concept of the evolution of the 
horse (Fig. 42). 

Further research showed that the situation was even more 
complex than that illustrated by Lull and in 1951 the scheme 
shown in Fig. 43 was a more accurate account of the evolution of 
the horse ; it will be noted that instead of a simple direct line the 
pattern has become more and more branched. 

To the interested non-specialist there are several things that are 
puzzling in the accounts of the evolution of the horse. In the first 
place it is difficult to find a critical account of the basic information. 
The accounts given by Piveteau (1958) and by Matthew (1926) 
are more concerned with the names of the intermediate forms and 
the basic trends of evolution. The account given by Simpson 
(1951) is of great interest and very readable but it is written for a 
wider audience. It is necessary to go back to the references given 
in Matthew (1926) or the papers of Matthew and Stirton (1930), 



146 



VERTEBRATE PALAEONTOLOGY 



C 

<u 
o 
o 
V> 

Q. 



c 
a> 
o 

.Q 

0_ 



Q) 

c 
a> 
u 
o 



CD 
C 
CD 

o 

J. 

b 



c 

CO 

o 
o 

UJ 



Equus 



Hipparion 



Pliohippus 



Hipparion 



Protohippus 




Onohippidion 



Hippidion 



Anchitherium 




Merychippus 



Hypohippus 



Miohippus 



Mesohippus 




Epihi 



Dpus 



Orohippus 



Hyracotherium = Eohippus 



Fig. 42. Evolution of the horse. The scheme shown here is more 
complex than that suggested by Kowalevsky. (From Lull 1918.) 

and Osborn (1905, 1918) to get any satisfaction concerning the 
fossils themselves. 

What does the sceptical reader hope to find out? It takes a 
great deal of reading to find out for any particular genus just how 
complete the various parts of the body are and how much in the 
illustrated figures is due to clever reconstruction. The early 
papers were always careful to indicate by dotted lines or lack of 
shading the precise limits of the reconstructions, but later authors 
are not so careful. Secondly it is difficult to find out just how many 
specimens of a given genus are available for study. Thus it is 
one thing to know that our information on Hyracotherium is 
based on, say, 500 specimens, and another if our information is 



VERTEBRATE PALAEONTOLOGY 



147 



<v 

c 

o 

o 




0) 

c 

o 

o 



8 

o 
o 
en 



a> 

c 

0) 

o 
o 

UJ 



Onohippidium Porahipporion 



Equus 



Stylohipparion 



Neohipparion 

Nannippus 

Hipparion 
Calippus 





Magahippus 



Archaeohippus 




Hypohippus 



Anchitherium 



Miohippus 
Mesohippus 



Epihippus 

Orohippus 

Hyracotherium 

II 

Fig. 43. Evolution of the horse. The scheme is more complex than 
that suggested by Lull. (After Simpson 1951.) 



based on five specimens. In the former case we have a very 
good idea of the form of the genus and the extent to which its 
characters overlap those of related genera (that is, provided that 
the 500 specimens are not just isolated cones of teeth!). A cer- 
tain amount of information concerning the number of fossil horses 
is available. Thus Simpson (1943) quotes the numbers of 
specimens of fossil horses in the American Museum of Natural 
History as follows : Lower Eocene 397 ; Middle Eocene 54 ; Upper 
Eocene 11; Lower Oligocene 30; Middle Oligocene 125; Upper 
Oligocene 39. The same author in his account of horses (1951) 
has an appendix " where to see fossil horses." He mentions that 
there are some fifty-two mounted skeletons of fossil horses in the 

11— IOE 



148 VERTEBRATE PALAEONTOLOGY 

U.S.A. and probably a total of 100 in the world. There are not 
any mounted skeletons of Eohippus> Archaeohippus, Megahippus, 
Stylohipparion, Nannippus Calippus, Onohippidium or Parahippus, 
and none in the United States of Anchitherium or Hipparion. 

There are, however, several thousand of horse fragments 
collected in the various museums of the world. It is expecting a 
great deal to have fully prepared specimens of all the major genera 
of fossil horses. But since the horse is such a key example in the 
evolutionary doctrine it is important that our knowledge of the 
fragments be collected, possibly in the first place as a card index 
system and then later published as a catalogue, so that the results 
can be made available in synoptic form to all those interested. 

A third problem concerns the validity of the various genera and 
generic differences. The number of genera described has in- 
creased considerably. Thus Kowalevsky in 1874 knew of three; 
Lull in 1917 described fifteen; Simpson in 1945 lists twenty-six 
genera. To some extent this is due to the discovery and description 
of new material but one wonders how valid these genera really 
are. 

Another problem concerns the dating of these genera. When 
Matthew worked out the time taken for the evolution of the horse, 
he came to the conclusion it would take some 45 million years. 
His calculation was a rough one but it provided a useful guide. 
Since then this calculation has been modified by a uranium dating 
which places the Eocene back to 60 million years. Over this 
60 million years we have had some twenty-six genera and a large 
number of species of fossil horse evolving and it would be of the 
greatest interest to know the relative positions of these animals 
to one another, together with some indication as to the accuracy 
of the relative dating. Thus if we could know the parts of, say, 
Mesohippus skeleton that have been found in perfect condition, 
the number of specimens, fragments and so on of Mesohippus 
that are available, the strata from which each of these was derived, 
the degree of contemporaneity of the strata plus or minus so 
many million years, then we should have no qualms in accepting 
the evidence presented to us. At present, however, it is a matter of 
faith that the textbook pictures are true, or even that they are the 
best representations of the truth that are available to us at the 
present time. 



VERTEBRATE PALAEONTOLOGY 149 

One thing concerning the evolution of the horse has become clear. 
The story of the evolution of the horse has become more and 
more complex as further material is collected, and instead of a 
simple family tree the branches of the tree have increased in size 
and complexity till the shape is now more like a bush than a tree. 
In some ways it looks as if the pattern of horse evolution might 
be even as chaotic as that proposed by Osborn (1937, 1943) for 
the evolution of the Proboscidea, where, " in almost no instance 
is any known form considered to be a descendant from any other 
known form; every subordinate grouping is assumed to have 
sprung, quite separately and usually without any known inter- 
mediate stage, from hypothetical common ancestors in the Early 
Eocene or Late Cretaceous " (Romer 1949). We now know that 
the evolution of the horse did not always take a simple path. In 
the first place it is not clear that Hyracotherium was the ancestral 
horse. Thus Simpson (1945) states, " Matthew has shown and 
insisted that Hyracotherium (including Eohippus) is so primitive 
that it is not much more definitely equid than tapirid, rhinocerotid, 
etc, but it is customary to place it at the root of the equid group." 

Similarly it is clear that though in general the horses did 
increase in size, certain genera such as Orohippus, Archaeohippus 
and Nannippus appear to have been smaller than their ancestors. 
Edinger (1948) from her studies of the casts of the skull and the 
brains of fossil horses has concluded that the brain surface of the 
early fossil horses was perfectly smooth and that the sulci have 
developed at a later date. This would indicate that any resem- 
blances that have been drawn between the sulci on the brain of 
the modern horse and those of other mammals are either due to 
convergent evolution or to homoiology. 

It is quite likely that further studies will show that the complex- 
ity of horse evolution will prove to be as great as that found in the 
Proboscidea, Rhinocerotidea or Camelidae. 



CHAPTER 10 



CONCLUSIONS 



What conclusions, then, can one come to concerning the validity 
of the various implications of the theory of evolution? If we go 
back to our initial assumptions it will be seen that the evidence is 
still lacking for most of them. 

(1) The first assumption was that non-living things gave rise to 
living material. This is still just an assumption. It is conceivable 
that living material might have suddenly appeared on this world 
in some peculiar manner, say from another planet, but this then 
raises the question, " Where did life originate on that planet? " 
We could say that life has always existed, but such an explanation 
is not a very satisfactory one. Instead, the explanation that non- 
living things could have given rise to complex systems having 
the properties of living things is generally more acceptable to most 
scientists. There is, however, little evidence in favour of 
biogenesis and as yet we have no indication that it can be per- 
formed. There are many schemes by which biogenesis could have 
occurred but these are still suggestive schemes and nothing more. 
They may indicate experiments that can be performed, but they 
tell us nothing about what actually happened some 1,000 million 
years ago. It is therefore a matter of faith on the part of the 
biologist that biogenesis did occur and he can choose whatever 
method of biogenesis happens to suit him personally; the evidence 
for what did happen is not available. 

(2) The second assumption was that biogenesis occurred only 
once. This again is a matter for belief rather than proof. It is 
convenient to believe that all living systems have the same 
fundamental chemical processes at work within them, but as 
has already been mentioned, only a few representatives from the 
wide range of living forms have so far been examined and even 

150 



CONCLUSIONS 151 

these have not been exhaustively analysed. From our limited 
experience it is clear that the biochemical systems within proto- 
plasm are not uniform, i.e. there is no established biochemical 
unity. Thus we are aware that there are systems other than the 
Embden-Meyerhof and the tricarboxylic cycles for the systematic 
degradation of carbohydrates; a total of six alternative methods 
being currently available. High-energy compounds other than 
those of phosphorus have been described; the number of vital 
amino-acids has gone up from twenty to over seventy; all these 
facts indicate that the biochemical systems may be very variable. 
The morphological systems in protoplasm, too, show consider- 
able variation. It is possible that some aspects of cell structure 
such as the mitochondria and the microsomes might have arisen 
independently on several distinct occasions. It is also probable 
that two or more independent systems have evolved for the 
separation of chromosomes during cell division. 

It is a convenient assumption that life arose only once and that 
all present-day living things are derived from this unique experi- 
ence, but because a theory is convenient or simple it does not 
mean that it is necessarily correct. If the simplest theory was 
always correct we should still be with the four basic elements — 
earth, air, fire and water! The simplest explanation is not always 
the right one even in biology. 

(3) The third assumption was that Viruses, Bacteria, Protozoa 
and the higher animals were all interrelated. It seems from the 
available evidence that Viruses and Bacteria are complex groups 
both of which contain a wide range of morphological and physio- 
logical forms. Both groups could have been formed from diverse 
sources so that the Viruses and Bacteria would then be an 
assembly of forms that contain both primitive and secondarily 
simplified units. They would each correspond to a Grade rather 
than a Subkingdom or Phylum. We have as yet no definite 
evidence about the way in which the Viruses, Bacteria or Protozoa 
are interrelated. 

(4) The fourth assumption was that the Protozoa gave rise to 
the Metazoa. This is an interesting assumption and various 
schemes have been proposed to show just how the change could 
have taken place. On the other hand equally interesting schemes 
have been suggested to show the way in which the Metaphyta 



152 CONCLUSIONS 

could have given rise to both the Protozoa and the Metazoa. 
Here again nothing definite is known. We can believe that 
any one of these views is better than any other according to the 
relative importance that we accord to the various pieces of 
evidence. 

(5) The fifth assumption was that the various invertebrate 
phyla are interrelated. If biogenesis occurred many times in the 
past and the Metazoa developed on several finite occasions then 
we might expect to find various isolated groups of invertebrates. 
If on the other hand biogenesis was a unique occurrence it should 
not be too difficult to show some relationship between all the 
various invertebrate phyla. 

It should be remembered, for example, that though there are 
similarities between the cleavage patterns of the eggs of various 
invertebrates these might only reflect the action of physical laws 
acting on a restrained fluid system such as we see in the growth of 
soap bubbles and not necessarily indicate any fundamental 
phylogenetic relationship . 

As has already been described, it is difficult to tell which are the 
most primitive from amongst the Porifera, Mesozoa, Coelenterata, 
Ctenophora or Platyhelminthia and it is not possible to decide 
the precise interrelationship of these groups. The higher 
invertebrates are equally difficult to relate. Though the concept 
of the Protostomia and the Deuterostomia is a useful one, the 
basic evidence that separates these two groups is not as clear cut 
as might be desired. Furthermore there are various groups such 
as the Brachiopoda, Chaetognatha, Ectoprocta and Phoronidea 
that have properties that lie between the Protostomia and the 
Deuterostomia. It is worth paying serious attention to the con- 
cept that the invertebrates are polyphyletic, there being more than 
one line coming up to the primitive metazoan condition. It is 
extremely likely that the Porifera are on one such side line and it is 
conceivable that there could have been others which have since 
died away leaving their progeny isolated; in this way one could 
explain the position of the nematodes. The number of ways of 
achieving a specific form or habit is limited and resemblances 
may be due to the course of convergence over the period of many 
millions of years. The evidence, then, for the affinities of the 
majority of the invertebrates is tenuous and circumstantial; not 



CONCLUSIONS 153 

the tvpe of evidence that would allow one to form a verdict of 
definite relationships. 

(6) The sixth assumption, that the invertebrates gave rise to the 
vertebrates, has not been discussed in this book. There are several 
good reviews on this subject. Thus Neal and Rand (1939) pro- 
vide a useful and interesting account of the various views that have 
been suggested to explain the relationship between the inverte- 
brates and the vertebrates. The vertebrates have been derived 
from the annelids, arthropods, nemerteans, hemichordates and the 
urochordates. More recently Berrill (1955) has given a detailed 
account of the mode of origin of the vertebrates from the urochord- 
ates in which the sessile ascidian is considered the basic form. 
On the other hand, almost as good a case can be made to show that 
the ascidian tadpole is the basic form and that it gave rise to the 
sessile ascidian on the one hand and the chordates on the other. 
Here again it is a matter of belief which way the evidence happens 
to point. As Berrill states, "in a sense this account is science fiction." 

(7) We are on somewhat stronger ground with the seventh 
assumption that the fish, amphibia, reptiles, birds and mammals 
are interrelated. There is the fossil evidence to help us here, 
though many of the key transitions are not well documented and 
we have as yet to obtain a satisfactory objective method of dating 
the fossils. The dating is of the utmost importance, for until we 
find a reliable method of dating the fossils we shall not be able to 
tell if the first amphibians arose after the first choanichthian or 
whether the first reptile arose from the first amphibian. The 
evidence that we have at present is insufficient to allow us to 
decide the answer to these problems. 

One thing that does seem reasonably clear is that many of the 
groups such as the Amphibia (Save Soderberg 1934), Reptilia 
(Goodrich 1916) and Mammalia appear to be polyphyletic grades 
of organisation. Even within the mammals there is the suggestion 
that some of the orders might be polyphyletic. Thus Kleinenberg 
(1959) has suggested that the Cetacea are diphyletic, the 
Odontoceti and the Mysticeti being derived from separate 
terrestrial stocks. (Other groups that appear to be polyphyletic 
are the Viruses, Bacteria, Protozoa, Arthropoda (Tiegs and 
Manton 1958), and it is possible that close study will show that 
the Annelida and Protochordata are grades too.) 



154 CONCLUSIONS 

In effect, much of the evolution of the major groups of animals 
has to be taken on trust. There is a certain amount of circum- 
stantial evidence but much of it can be argued either way. Where, 
then, can we find more definite evidence for evolution? Such 
evidence will be found in the study of modern living forms. It 
will be remembered that Darwin called his book The Origin of 
Species not The Origin of Phyla and it is in the origin and study of 
the species that we find the most definite evidence for the evolution 
and changing of form. Thus to take a specific example, the 
Herring Gull, Larus argentatus, does not interbreed with the 
Lesser Black-backed Gull, Larus fuscus, in Western Europe, the 
two being separate species. But if we trace L. argentatus across 
the northern hemisphere through North America, Eastern Siberia 
and Western Siberia we find that in Western Siberia there is a form 
of L. argentatus that will interbreed with L. fuscus. We have here 
an example of a ring species in which the members at the ends of 
the ring will not interbreed whilst those in the middle can. The 
separation of what was possibly one species has been going on 
for some time (in this case it is suggested since the Ice Age). 
We have of course to decide that this is a case of one species 
splitting into two and not of two species merging into one, but 
this decision is aided by the study of other examples such as those 
of small mammals isolated on islands, or the development of 
melanic forms in moths. Details of the various types of speciation 
can be found in the books by Mayr, Systematics and the Origin 
of Species (1942), and Dobzhansky, Genetics and the Origin of 
Species (1951). 

It might be suggested that if it is possible to show that the 
present-day forms are changing and the evolution is occurring 
at this level, why can't one extrapolate and say that this in effect 
has led to the changes we have seen right from the Viruses to the 
Mammals? Of course one can say that the small observable 
changes in modern species may be the sort of thing that lead to 
all the major changes, but what right have we to make such an 
extrapolation? We may feel that this is the answer to the problem, 
but is it a satisfactory answer? A blind acceptance of such a view 
may in fact be the closing of our eyes to as yet undiscovered factors 
which may remain undiscovered for many years if we believe that 
the answer has already been found. 



CONCLUSIONS 155 

It seems at times as if many of our modern writers on evolution 
have had their views by some sort of revelation and they base 
their opinions on the evolution of life, from the simplest form to 
the complex, entirely on the nature of specific and intra-specific 
evolution. It is possible that this type of evolution can explain 
many of the present-day phenomena, but it is possible and indeed 
probable that many as yet unknown systems remain to be dis- 
covered and it is premature, not to say arrogant, on our part 
if we make any dogmatic assertion as to the mode of evolution 
of the major branches of the animal kingdom. 

Perhaps it is appropriate here to quote a remark made by 
D'Arcy Thompson in his book On Growth and Form. " If a tiny 
foraminiferan shell, a Lagena for instance, be found living today, 
and a shell indistinguishable from it to the eye be found fossil in 
the Chalk or some still more remote geological formation, the 
assumption is deemed legitimate that the species has ' survived ' 
and has handed down its minute specific character or characters 
from generation to generation unchanged for untold millions of 
years. If the ancient forms be like rather than identical with 
the recent, we still assume an unbroken descent, accompanied by 
hereditary transmission of common characters and progressive 
variations. And if two identical forms be discovered at the 
ends of the earth, still (with slight reservation on the score of 
possible ' homoplasy ') we build a hypothesis on this fact of 
identity, taking it for granted that the two appertain to a common 
stock, whose dispersal in space must somehow be accounted for, 
its route traced, its epoch determined and its causes discussed or 
discovered. In short, the Naturalist admits no exception to the 
rule that a natural classification can only be a genealogical one, 
nor ever doubts that ' the fact that we are able to classify organ- 
isms at all in accordance with the structural characteristics which 
they present is due to their being related by descent.' " 

What alternative system can we use if we are not to assume 
that all animals can be arranged in a genealogical manner? The 
alternative is to indicate that there are many gaps and failures 
in our present system and that we must realise their existence. 
It may be distressing for some readers to discover that so much 
in zoology is open to doubt, but this in effect indicates the vast 
amount of work that remains to be done. In many courses the 



156 CONCLUSIONS 

student is obliged to read, assimilate and remember a vast amount 
of factual information on the quite false assumption that know- 
ledge is the accumulation of facts. There seems so much to be 
learnt that the only consolation the student has is that those who 
come after him will have even more to learn, for more will be 
known. But this is not really so ; much of what we learn today are 
only half truths or less and the students of tomorrow will not be 
bothered by many of the phlogistons that now torment our brains. 
It is in the interpretation and understanding of the factual 
information and not the factual information itself that the true 
interest lies. Information must precede interpretation, and it is 
often difficult to see the factual data in perspective. If one reads 
an account of the history of biology such as that presented by 
Nordenskiold (1920) or Singer (1950) it sometimes appears that 
our predecessors had a much easier task to discover things than we 
do today. All that they had to do was realise, say, that oxygen 
was necessary for respiration, or that bacteria could cause 
septicaemia or that the pancreas was a ductless gland that secreted 
insulin. The ideas were simple; they just required the thought 
and the experimental evidence ! Let us have no doubt in our minds 
that in twenty years or so time, we shall look back on many of 
today's problems and make similar observations. Everything will 
seem simple and straightforward once it has been explained. Why 
then cannot we see some of these solutions now? There are many 
partial answers to this question. One is that often an incorrect 
idea or fact is accepted and takes the place of the correct one. An 
incorrect view can in this way successfully displace the correct 
view for many years and it requires very careful analysis and much 
experimental data to overthrow an accepted but incorrect theory. 
Most students become acquainted with many of the current 
concepts in biology whilst still at school and at an age when most 
people are, on the whole, uncritical. Then when they come to 
study the subject in more detail, they have in their minds several 
half truths and misconceptions which tend to prevent them from 
coming to a fresh appraisal of the situation. In addition, with a 
uniform pattern of education most students tend to have the same 
sort of educational background and so in conversation and dis- 
cussion they accept common fallacies and agree on matters based 
on these fallacies. 



CONCLUSIONS 157 

It would seem a good principle to encourage the study of 
" scientific heresies." There is always the danger that a reader 
might be seduced by one of these heresies but the danger is 
neither as great nor as serious as the danger of having scientists 
brought up in a type of mental strait-jacket or of taking them so 
quickly through a subject that they have no time to analyse and 
digest the material they have " studied." A careful perusal of the 
heresies will also indicate the facts in favour of the currently 
accepted doctrines, and if the evidence against a theory is over- 
whelming and if there is no other satisfactory theory to take its 
place we shall just have to say that we do not yet know the answer. 

There is a theory which states that many living animals can be 
observed over the course of time to undergo changes so that new 
species are formed. This can be called the " Special Theory of 
Evolution " and can be demonstrated in certain cases by experi- 
ments. On the other hand there is the theory that all the living 
forms in the world have arisen from a single source which itself 
came from an inorganic form. This theory can be called the 
" General Theory of Evolution " and the evidence that supports 
it is not sufficiently strong to allow us to consider it as anything 
more than a working hypothesis. It is not clear whether the changes 
that bring about speciation are of the same nature as those that 
brought about the development of new phyla. The answer will 
be found by future experimental work and not by dogmatic 
assertions that the General Theory of Evolution must be correct 
because there is nothing else that will satisfactorily take its place. 



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AUTHOR INDEX 



Ahrens, L. H. 137, 140 

Amano, S. 12 

Aristotle, 14 

Baker, J. R. 35, 44, 46, 47, 48 

Baldwin, E. 112-124, 128 

Balfour, F. M. 69 

Beatty, I. M. 128 

de Beer, G. 46, 79, 93, 96 

van Beneden, E. 71 

Bergman, W. 130-133 

Bernal, J. D. 8 

Berrill, N. 153 

Borsuk, V. N. 121 

Bonner, J. T. 44 

Bouillon, J. 56 

Boyden, A. 12, 13 

Bresslau, E. 99 

Bunting, M. 31 

Butschli, O. 32 

Carlgren, O. 87 

Castiaux, P. 56 

Caullery, M. 72, 75 

Chargaff, I. 20 

Chatton, E. 41 

Claus, F. W. 66 

Cohen, S. 10 

Cooper, G. A. 143 

Cuenot. L. 101 

Darwin, C. 4, 154 

Dawydoff, C. 103 

Delage, Y. 29, 69 

DOBZHANSKY, T. 154 

Dodson, E. O. 72 
Doflein, F. 33 
Dogiel, V. 41 
Duboscq, O. 41, 59 
Edinger, T. 149 
Eggleton, P. and G. P. 113 



Ennor, A. H. 124, 127 

Faure-Fremiet, E. 43 

Fraenkel-Conrat. H. 19 

Franz, V. 33, 47 

Fruton, J. S. 20, 115 

Fry, B. A. 22 

Goette, A. 101 

Goodrich, E. S. 153 

von Graff, L. 99 

Grasse, P. P. 12, 24, 25, 29, 33 

Gray, J. 15 

Grobben, K. 101-106, 110 

Grontved, J. 59 

Gurwitsch, A. 15 

Hadzi, J. 21, 47, 79, 81, 83, 85, 

93-99, 108-110 
Haeckel, E. 62, 63, 64, 65, 66, 79, 

81, 83, 87 
Haldane, J. B. S. 8, 17 
Hand, C. 87 
Hardy, A. C. 48 
Harper, H. 115 
Heath, H. 96 
Hedley, R. D. 65 
Herouard, E. 29 
Hershey, A. D. 19 
Hobson, G. E. 124 
Hollaender, A. 15 
Hovasse, R. 30 
Huxley, T. H. 76-78 
Hyman, L. H. 29, 33, 61, 68, 71, 80, 

81, 121, 129 
Ishii, M. 19 
Ivanov, I. V. 83 
Jagersten. G. 66, 67, 68, 81, 83 
Jarvik, E. 135 
Jeans, J. 16 
Johnstone, G. 76 



169 



170 



AUTHOR INDEX 



Keilin, D. 9 
Klebs, G. 29 
Knopf, A. 137 
Kleinenberg, S. 153 
Komai, T. 85, 89 

KORNBERG, H. L. 10 

Kowalevski, V. O. 145, 148 
Krebs, H. 9, 10 
Kreps, E. 121 
Krumbach, T. 29, 87 

KUKENTHAL, W. 29 

Kulp, J. L. 137, 138, 139 

Lambert, R. St. J. 140 

Lang, A. 89, 90, 91 

Lankester, E. R. 27, 29, 30, 65, 69 

Lea, D. 18 

Lederberg, J. 22 

Leuchtenberger, C. 12 

LlPMANN, F. 12 

Lull, R. 145, 148 

Lwoff, A. 30 

Luria, S. E. 21 

Lynen, F. 12 

Lyttelton, R. 16 

Manton, S. M. 103, 105, 111, 153 

Marcus, E. 81, 99, 106-108, 110 

Markham, R. 18 

Margrath, K. I. 127 

Matthew, W. D. 139, 145 

Mayne, K. I. 140 

Mayr, E. 154 

McConnaughey, B. H. 74 

Meister, A. 1 1 

Meyerhoff, O. 114 

Moore, R. C. 80 

Morrison, J. F. 124, 127 

Moser, F. 81 

Murphey, B. F. 137 

Neal, H. V. 153 

Needham, D. M. 114-119 

Needham, J. 15, 114-119, 128 

Nier, A. O. 137 

Nordenskiold, E. 156 

O'Kane, D. J. 23 

Oparin, A. I. 8, 14, 16, 17, 23, 25 

Osborn, H. F. 146, 149 

Ousdal, A. P. 16 

Paley, W. 2 

Pantin, C. F. A. 81 

Peel, J. 22 

Picard, J. 85 



Pirie, A. 8 

PlVETEAU, J. 145 

Pocock, M. A. 60 

Pradel, L.-A. 124-128 

Pringle, J. W. S. 8 

Pringsheim, E. G. 29, 30 

Prosser, C. L. 112 

Radl, E. 5, 66 

Ralph, P. M. 87 

Rand, H. W. 153 

Rasmont, R. 56 

Raven, Ch. 106 

Rees, K. R. 124 

Reich, W. 15 

Remane, A. 81, 106 

Robin, Y. 124-128 

Robinow, C. F. 22 

Roche, J. 124-128 

Romer, A. S. 149 

Save Soderberg, G. 135, 153 

Saville Kent, W. 57, 65, 69 

SCHRADER, F. 12 
SCHOEFFEL, E. 15 

Seaman, G. R. 128 

Sedcwick, A. 103, 104, 106 

Shapley, H. 17 

Simmons, S. 20, 115 

Simpson, G. G. 142, 144, 145, 146, 

147, 149 
Singer, C. 14, 156 
Smith, J. D. 18 
Smith, K. M. 18 
Spooner, E. T. C. 22 
Stirton, R. A. 145 
Stocker, B. A. D. 22 
Stunkard, H. 72 
Summers, F. M. 43, 48 
Swann, M. M. 12 
Takahashi, W. N. 19 
Thiem, N. 124-126 
Thoai, Ng-V. 124-128 
Thompson, R. W. 137 
Thompson, W. D'Arcy, 155 
Tiegs, O. Ill, 153 
Tuzet, O. 59, 61 
Ulrich, W. 27,44,81, 106 
Verbinskaya, N. A. 121 
Vendermeerssche, G. 56 
Virgil, 14 
Wang, Y. L. 9 



AUTHOR INDEX 



171 



Wheeler, L. R. 14 
Williams, A. 143, 144 
Williams, R. 19 
Willmer, E. N. 31, 44, 49 

WOHLER, F. 7 



York, D. 140 
Yudkin, J. 114-119 
Yudkin, W. H. 122, 123, 124 
Zeuner, F. 137 
Zinsser, H. 22 



SUBJECT INDEX 



Acoela, 83, 95, 98-100 

Aequorea, 85 

Alcyonium, 79 

Algae, relation to Protoza, 33-35 

relation to Metazoa, 46-48 
Ameria, 94, 110 
Amino acids, 11 
Amoeba, 26, 35 
Amphioxus, 65, 83, 97, 119 
Amphipholis, 126 
Ammonia, 127 
Annelid superphylum, 105 
Antedon, 121 
Anthea, 116, 117, 119 
Anthosigmelia, 132 
Anthozoa, 68, 79, 81, 82, 83, 93 
Antipathes, 82 
Anus, 68, 102-105 
Apis, 126 
Aphrodite, 126 
Arbacia, 12, 124, 131 
Archigastrula, 66 
Articulata, 108 

Arginine phosphate, 112 et seq. 
Aschelminthes, 108 
Ascidia, 117, 119 
Assumptions, 6 et seq., 150 et seq. 
Astaciis, 102 
Astasia, 29, 30 
Asterias, 117, 119, 131 
Amelia, 76, 79, 114 
Autotrophic bacteria, 22-23, 25 
Bacteria, relation to Protozoa, 24-25, 
151 



Bacteriophage, /aa'ng 18, 19 

Balanoglossus, 117, 119, 120, 124 

Bilateria, 106-107 

Bilateral symmetry, 52-54, 81, 90 

Biogenesis, 7, 150-151 

Bions, 15 

Biogenetic fundamental law, 65 

Blastopore formation, 101-105 

Blastula, 59, 69, 70, 83, 96 

Bombyx, 126 

Bodoines, 41 

Bonellia, 72 

Calliactis, 127 

Cambridge, 1 

Camelidae, 149 

Capitella, 102 

Carchesium, 41 

Caridina, 102 

Cellularisation, 43 

Centrostephanus, 124 

Ceratium, 40 

Chemoautotrophs, 22, 23, 24 

Chlamydomonas, 26, 29, 36, 37, 47, 95 

Chlorophyll, 31 

Choanoflagellata, 57 

Choanocytes, 54, 55, 70 

Cholesterol, 131 

Chonotricha, 27 

Chordonia, 110 

Christianity, 2, 4 

Church, 1, 2, 3 

Cleavage, in Ctenophores 

radial, 92 

spiral, 92 
Clymene, 127 



172 



SUBJECT INDEX 



Clytia, 83 

Cnidaria, 85-87 

Coelenterata, 76; Complex characters, 
78-79, 96-97 
simple characters, 78 
most primitive, 79-82 
relation to Ctenophora, 84-87 
relation to Porifera, 69-70 
aberrant forms, 87, 89 

Coelom, 105 

Coelomata, 104, 106 

Coeloplana, 54, 84, 89, 90-94 

Colonies, 36 et seq. 

Convoluta, 93, 102 

Corallium, 78 

Ctenodrilus, 102 

Ctenophora, ancestry, 84, 95 
coelenterate affinities, 76, 84-87 
platyhelminth affinities, 89-94 

Ctenoplana, 54, 88, 90-94 

Cucumaria, 117, 121, 131 

Cunina, 85, 86 

Cytula, 62, 63 

DNA (deoxyribosenucleic acid), 12, 
19 

Daphnia, 9 

Dendrobaena, 102 

Deuterostomia, 102-104 

Dictyostelium, 44 

Dicyema, 72, 73, 75 

Dicyemidae, 72, 74-75 

Dimorpha, 31 

Dinoclonium, 34, 35 

Dinofiagellates, 33, 41 

Diplodinium {Entodinium) , 26, 27, 28 

Earliest fossils, 134 

Echinocardium, 117 

Echinoderm, relationships, 52, 124, 
129 
superphylum, 105 

Ectoprocta, 76, 152 

Elasmobranchs, 135, 137 

Embden-Meyerhof cycle, 9, 10, 151 

Enantiazoa, 69 

Entodinium {Diplodinium), 26, 27, 28 

Equus, 145, 147 

Euchlora, 84, 85, 86, 87 

Eudorina, 37, 38, 40 

Euglena, 29, 30 

Euglypha, 65 

Euplectella, 56 



Evolution, definition, 6-7 

Bacteria, 22-25 

Brachiopoda, 144 

Carnivora, 142-144 

horse, 144-149 

Lamellibranchs, 142-143 

life, 13 et seq. 

Mesozoa, 50 

Metazoa, 36 et seq., 50 et seq. 

Platyhelminthia, 50, 94 

Porifera, 54-71 

Protozoa, 24-25, 32-35 

vertebrates, 134 et seq. 

viruses, 21 
Foraminifera, 33, 44, 65 
Fossils, 134-137, 145-149 
Fossil stratification, 137-142 
Gastrodes, 85, 87 
Gastrula, 65-68, 69, 97 
Germ layers, 55, 56, 60, 61, 66-68 
General theory of Evolution, 7, 157 
Giardia, 46 

Glycocyamine, 121, 125 
Glycocyamine phosphate, 125-128 
Glycera, 123, 127 
Gonactinia, 116 
Gonium, 36, 38 
Grade, 21, 35 
Grafizoon, 96 
Grantia, 59-61 
Gymnodinium, 34, 35, 40 
Haemoglobin, 8 
Halichondria, 127, 133 
Haliclystus, 80 
Haliphysema, 62, 63-64, 65 
Haplozoon, 41, 72 
Heliocidaris, 124, 131 
Heterotropic bacteria, 23-25 
Hermione, 126 
High energy bonds, 11, 12 
Hirudo, 127, 128 
Hitodestrol, 131 
Holothuria, 114, 121, 131 
Hormiphora, 89 

Horses, evolution of, 144 et seq. 
Hydra, 11, 79, 81, 82, 83 
Hydroctena, 85, 87, 89 
Homoiology, 108, 136 
Hydrozoa, 68, 76, 77, 79-83, 87, 

94-96 
Hyracotherium (Eohippus), 146-149 



SUBJECT INDEX 



173 



Implications, 150 et seq. 
Ischnochiton, 103 
Inversion, of Grantia, 59-61 

of Volvox, 60-61 
Invertebrate phyla, 101 
Invertebrate phylogeny, 101 et seq. 
Lampetia, 89 
Lead, 137-139 
Leacosolenia, 69 
Life, on other planets, 17 

origin of, 8 et seq., 13 et seq. 
Lineus, 116, 117, 119, 127 
Lingula, 143-144 
Limnaea, 126 
Lohmann reaction, 114 
Lombricine, 128 
Lucernaria, 80 
Lumbriconereis, 127 
Lumbricus, 114, 122, 127, 128 
Maia, 126 
Marphysa, 127 
Martasterias, 126 
Mastigamoeba, 31, 33 
Medusa, 79, 80, 83, 87, 96 
Melicerta, 54 
Mesozoa, 41, 50, 71-76, 152 

resemblances to Trematodes, 74 
Meteorites, 16, facing 16, facing 17 
Metaphyta, 13, 46, 47 
Metazoa, origin of, 25, 36 et seq., 
151-152 
most primitive, 50 et seq., 82-84 
Mitosis, 12 

Miracidium, 72, 74, 76 
Monerula, 62, 63 
Monocystis, 26, 44 
Mouth, 68, 102-105 
Mycetozoa, 27 
Mytilus, 126 
Myxicola, 126 
Myxobolus, 43, 45 
Muller's larva, 90, 96 
Naegleria, 31, 44, 49 
Natural selection, vii 
Nematocysts, 81 
Nematoda, 106, 129 
Nemertini, 116, 117, 129 
Nephthys, 126, 127 
Nereis, 102, 116, 117, 118, 119, 120, 

122, 123, 125, 126, 127 
Nucleic acids, 20 



Nucleoproteins, 8 
Obelia, 78 

Octopus, 72, 117, 118 
Oligomeria, 110 
Opalina, 33, 46, 47 
Ophioderma, 121 
Ophiothrix, 126 
Origin of, Bacteria, 22-25 
Coelenterates, 76-84 
life, 7, 150-151 
Metazoa, 36 et seq. 
Porifera, 54 
Protozoa, 26 et seq. 
vertebrates, 120, 124, 134 et seq. 
viruses, 18-21 
Origin of Phyla, 154 
Origin of Species, 4-5, 154 
Orgones, 15 

Orthonectidae, 72, 74-75 
Osteichthyes, 137 
Ostrea, 126 

Palaeontology, 134 et seq. 
Palaeotherium, 145 
Palmella stage, 37, 47 
Paramecium, 26, 46 
Pecten, 114 
Pegmatites, 138, 139 
Peripatus, 102, 103 
Pheopolykrikos, 40, 44 
Phoronidea, 107 
Phosphagens, Protozoa, 129 
Coelenterate, 116 
sponges, 129 
Platyhelminth, 112, 116 
annelid, 112, 116, 122-123 
cephalopod, 112 
echinoderm, 113, 120-122 
protochordate, 113, 123 
vertebrate, 113, 122 
Phosphorus, 113 et seq. 
Physalia, 76, 79 
Physemaria, 63, 64 
Planaria, 116, 117, 119 
Planocera, 102 
Planula larva, 69, 70, 83, 87 
Plasmodiophora, 46 
Plas?nodium, 26, 33 
Plasmodroma, 33 

Platyhelminthes, 50, 67, 72, 89, 94-96, 
99-100, 106-107, 116, 129 



174 



SUBJECT INDEX 



Pleodorina, 37, 40 

Pleurobrachia, 76, 84, 86, 89, 116, 117, 

119 
Podarke, 102 
Pogonophora, 56, 83, 106 
Polycelis, 116, 117, 119 
Polycladida, resemblances to Cteno- 

phora, 90-94 
Polykrikos, 40 
Polymeria, 110 
Polyp, 79, 80, 83, 96, 97 
Polyphyletic, 13, 152, 153 
Pomatoceros, 102 
Porifera, 50, 54-71, 82, 132, 152 
Porpita, 79 
Potassium, 140 
Poterion, 56 
Primitiveness, 51, 54 
Proboscidea, 149 
Protanthea, 82 
Proterospongia, 57 
Protostomia, 102-105 

Protozoa, origin, 24-25, 46 

colonial forms, 36-44, 47, 57 et seq. 
interrelationship, 32-35 
most primitive, 26, 47 
syncytia, 44-47 

Protista, 13 

Pterobranchiata, 83 

Radial symmetry, 52, 96 

Radioactive dating of rocks, 137-140 

Rates of evolution, 142-144 

Rhinoscerotoidea, 149 

Rhizoflagellata, 33 

Rhopalura, 72, 75 

Rickettsia, 21-22 

Sabella, 54, 123, 127 

Sabellaria, 116, 117, 119, 123 

Saccocirrus, 102 

Sacculina, 51 

Sakaguchi's test, 120, 121 

Salpa, 87 

Sappinia, 46 

Scolecida, 102 

Scyphozoa, 68, 78, 79, 80, 81, 82, 84, 
87, 93-96 

Sepia, 117, 118, 119, 120, 126 

Seymouria, 136 

Simplicity, 51 

Siphonophora, 76 

Sipunculus, 114, 117, 118, 127 



Special theory of Evolution, 157 
Spaer echinus, 121, 126 
Spiral cleavage, 92, 105 
Spirochona, 27 
Spirographs, 116, 117, 123 
Sponges, origin, 57, 71 

specialised characters, 55-56 

simple characters, 55 
Spongiaria, 94 
Sporozoa, 26, 43, 44, 45, 53 
Stellasterol, 130, 131 
Sterols, 129-133 

Porifera, 132 

Echinodermata, 129, 133 
Stichopus, 114 

Stronglylocentrotus, 117, 119, 124 
Stylotella, 132, 133 
Suberites, 132 
Sulphur, 12, 23 
Symmetry, 52-54, 90, see also Radial 

symmetry; Bilateral symmetry 
Synapta, 117, 119 
Syncytia, 39, 44, 46, 63 
Taurocyamine, 125 
Taurocyamine phosphate, 125-128 
Teredo, 103 
Tetrahymena, 127, 128 
Tetramitus, 31 
Tetraplatia, 87, 89 
Tentaculata, 107 
Thetia, 126 
Thiobacillus , 11, 23 
Thorium, 138 
Trachylina, 78, 84 
Trematodes, 72, 94 
Tricarboxylic acid cycle, 9, 10, 11 
Trichonympha, 26 
Trochosphaera, 54 
Turbellaria, 83, 89-94, 108 
Tubularia, 76, 77 
Uranium, 137-139 
Velella, 79 
Vermes, 35 
Vertebrata, 102, 113, 129, 134 et seq., 

153 
Viruses, 18-21, 151 
Volvox, 37, 39, 40, 44, 46, 57, 59, 

60-61 
William of Occam, 9 
Zoothamnion, 41, 42, 48